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Optics For Practical Long Range Rifle Shooting

Discussion in 'Rifle Country' started by Zak Smith, Sep 1, 2005.

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  1. Zak Smith

    Zak Smith Moderator Emeritus

    Dec 24, 2002
    Fort Collins, CO, USA.
    This is Part II of a three-part series:

    article | Practical Long-Range Rifle Shooting, Part I - Rifle & Equipment extwh3.png

    article | Practical Long-Range Rifle Shooting, Part II - Optics extwh3.png

    article | Practical Long-Range Rifle Shooting, Part III - Shooting extwh3.png


    (C) Copyright 2005 Zak Smith All Rights Reserved

    Reproduction or Republication by express written permission only


    (Schmidt & Bender PMII scope with 13 mils elevation, 0.1mil clicks, on a Remington PSS rifle.)

    What is Practical Precision / Long Range Rifle Shooting?

    Practical precision rifle shooting involves engaging small and/or distant targets at the limit of
    weapon, ammunition, and shooter capability under time pressure in field settings.

    Applications include but are not limited to: very small targets 1/4"-1" at 100 to 200 yards,
    so-called "cold bore" shots, arbitrary unknown distance targets, shooter/spotter communication, and
    combinations of all of those under time constraints.

    Generally, these include everything a rifleman is likely to find in any "sniper", "tactical", or
    "field" rifle match. The typical platform is a bolt action rifle, though an autoloader of
    sufficient accuracy and appropriate caliber can do the job with some tradeoffs.

    For our purposes, consider "long range" to mean within a few hundreds yards of the load's
    trans-sonic boundary (the point at which the bullet slows to the speed of sound, Mach 1). For
    example, with typical 308 loads and rifles, we are interested in ranges from 25 yards out to about
    700-1000 yards.

    Ballistics Background

    Some understanding of bullet trajectory and the physical factors affecting bullet flight is needed
    as background before discussing optics.

    In the simplest case, take an accurate rifle with sights zeroed at 100 yards shooting one type of
    ammunition. In the absence of wind or shooter error, the bullet will impact the point of aim (POA)
    when the target distance is 100 yards-- hence its "zero" is at 100 yards.

    The "line of aim" is a line straight from the shooter's eye, through the sighting device, to the
    target. The bullet starts off below the LOA by the distance between the center of the sighting
    device and the center of the bore. This is called the "sight over bore" distance. The axis of the
    bore is not parallel to the LOA-- the bore is angled slightly upwards. This causes the bullet to
    start off with some "upward" velocity. As it flies down-range, it rises to meet the point of aim
    (POA) which is where the LOA intersects with the target.

    Depending on the bullet's velocity, the bullet might keep rising above the LOA and again intersect
    with it a second time as it falls. Alternatively, it may rise just enough to meet the LOA and then
    start to fall again.


    In this graph, two loads are displayed. The green trajectory is a 308 load zeroed at 100 yards. It
    starts 2" low, rises to the LOA at 100 yards, and then drops off, ending around 11.5" low at 300

    The red trajectory is the same load zeroed at 200 yards. It starts 2" low, intersects with the LOA
    the first time at about 40 yards. At 120 yards, it's about 1.6" above the LOA, then drops,
    intersecting the LOA again at 200 yards. This is the second, or primary, zero. At 300 yards, it's
    about 7" low.

    Looking at the graph with the 200 yard zero, the point of impact (POI) at 100 yards would be about
    1.6" above the point of aim (POA). At 240, the POI will be 2" below the POA. At 300, the POI will
    be 7.5" below the POA. Thus, to hit a small target at 300 yards, the shooter would have to hold 7"
    above the target. The bullet continues to fall relative to the line of aim as target range is

    A table can be constructed which relates the drop distance for every range out to the maximum
    engagement range. An abbreviated table might look like this, for a rifle with a 100 yard zero. (An
    actual table would have intermediate distances like 120, 140, etc.)
     100     0"
     200     2.87"
     300     11.2"
     400     25.6"
     500     46.9"
     600     76.0"
     700    114.9"
     800    161.7"
    This is helpful, but the shooter is left with the problem of how to aim 47" higher than the target
    when the distance is 500 yards. There won't be a 47" yardstick sticking out above the target.
    Aiming the cross-hairs at a point imagined to be 47" above the target is difficult and very error

    Angular measurements

    Instead of measuring hold-over in terms of linear distance (inches or cm), it would be helpful to
    translate those linear distances into units of angular measure. The concept of angular measure is
    that an angle of 1 degree demarcates 1.7 yards at 100 yards, or 3.5 yards at 200 yards. Everyone
    with a basic understanding of geometry should understand how angles work.

    There are two units of angular measurement commonly used in rifle scopes. The first is the "minute
    of angle." Dividing a circle into 360 degrees, then each degree contains 60 minutes. One MOA
    demarcates 1.0472" per 100 yards of distance.

    The second is the "mil". One mil is one part transverse per 1000 parts distance. In units we
    understand, 1 mil is 3.6" per 100 yards (ie, 100 yards is 3600", one thousandth of which is 3.6").
    Consequently it's also 1 yard at 1000 yards. Alternatively, in metric, 1 mil is 10cm per 100
    meters, or 1m at 1000 meters.


    Just like the atmosphere pushes on the bullet as it moves forward, slowing it down, any winds
    present in the bullet's path can affect its trajectory. The most common effect is the cross wind. A
    10mph cross wind will move a typical 308 bullet about 6" at 300 yards. The following graph
    demonstrates the wind deflection as range increases for a left or right 10mph wind.


    Just like the drop table, we can generate a wind table, which might look something like this:
    RANGE  DRIFT for 10mph cross
     100    0.6"
     200    2.6"
     300    6.0"
     400   11.0"
     500   17.8"
     600   26.5"  
     700   37.5"
     800   50.9"

    For moving targets, the shooter must aim in front of the target a distance which depends on the
    target distance and speed. This is called "lead." We'll generate a table for some standard target
    speed and add it to our table.

    Both the "drift" and "drop" values in the tables can be translated to use angular measurements (MOA
    or mils) instead of linear measurements (inches or cm) to aid utility.

    Typical Data Card

    The shooter might end up with a data card that looks something like this. The first line describes
    the load so he can keep straight what the data-card describes. The second line reminds him what
    each column means.
    155 LAP: 2825fps 100yd 0'
    RANGE   elev wind   4mph->(MOA)
      25    4.00  0.25   6 moa
      50    0.75  0.25   6 moa
      75    0.00  0.50   6 moa
     100    0.00  0.50   6 moa
     125    0.25  0.75   6 moa
     150    0.50  1.00   6 moa
     175    1.00  1.00   6 moa
     200    1.50  1.25   7 moa
     225    2.00  1.50   7 moa
     250    2.50  1.50   7 moa
     275    3.00  1.75   7 moa
     300    3.75  2.00   7 moa
     325    4.25  2.00   7 moa
     350    5.00  2.25   7 moa
     375    5.75  2.50   7 moa
     400    6.50  2.50   7 moa
     425    7.25  2.75   7 moa
     450    8.00  3.00   7 moa
     475    8.75  3.25   7 moa
     500    9.50  3.50   7 moa
     525   10.25  3.50   7 moa
     550   11.25  3.75   7 moa
     575   12.00  4.00   7 moa
     600   13.00  4.25   8 moa
     625   13.75  4.50   8 moa
     650   14.75  4.75   8 moa
     675   15.75  5.00   8 moa
     700   16.75  5.00   8 moa
     725   17.75  5.25   8 moa
     750   18.75  5.50   8 moa
     775   20.00  5.75   8 moa
     800   21.00  6.00   8 moa

    1. Range

    2. elevation for #1's target distance, in MOA

    3. wind for #1's target distance, in MOA

    4. lead for #1's target distance, in MOA for a target traveling at 4mph (a medium walking pace)

    All the trajectory values can be calculated using one of the modern small-arms ballistics calculator
    programs, such as Sierra Ballistic Explorer, Exbal, QuickTarget, Agtrans, etc. Several parameters
    are critical to their accuracy: (1) bullet ballistic coefficient (BC) values, (2) accurate measured
    muzzle velocity from a chronograph, (3) solid zero distance, and (4) accurate environmental
    conditions including station pressure, temperature, or density altitude.

    Data Confirmation by Shooting

    It is important to verify computed data by actually shooting targets at various distances and
    looking at the actual hits (or misses) to determine if the elevation values are correct. Shooting
    known-distance targets every 100 yards out to the maximum range is a good way to do this.

    Desired Sighting System Capabilities

    Let's look at the things we want to accomplish with the rifle sighting system:

    1. Precisely specify drop hold-over out to our maximum engagement distance.

    2. Precisely specify wind drift out to our maximum engagement distance.

    3. Precisely specify target lead for moving targets/shooter.

    4. Range targets of known size when Laser Range-finders are not appropriate

    5. Observe target area

    6. Retain #1-5's capabilities in low light conditions

    Optical Considerations

    Magnified rifle optics have several salient optical properties which we need to understand before
    discussing the capability trade-offs later:

    Parallax Error

    Parallax is the error in apparent POA vs. actual POA due to misalignment of the shooter's eye
    vs. the scope's axis. A scope can be set to be parallax error free at one distance. A scope
    either has adjustable or fixed parallax. Fixed parallax means the distance at which there is no
    error is fixed to something like 100 or 200 yards from the factory. Most tactical scopes have
    adjustable parallax, which means the user can adjust the parallax error free distance on the fly
    to reduce parallax error whatever the current target's distance.

    First Focal Plane vs. Second Focal Plane Definition

    Variable-magnification optics can have a first focal plane (FFP) or second focal plane (SFP) reticle
    configuration. A first-focal (FFP) reticle's features always demarcate the same angular
    measurement regardless of the scope magnification setting. The reticle will appear to "shrink"
    and "grow" with the target area as the magnification is adjusted.

    A second focal plane (SFP) reticle demarcates angular distance that depends on the scope
    magnification setting. The reticle appears to stay constant as the target area shrinks and grows
    as the magnification is adjusted.

    A fixed power optic is FFP by definition.

    Last edited: Feb 20, 2008
  2. Zak Smith

    Zak Smith Moderator Emeritus

    Dec 24, 2002
    Fort Collins, CO, USA.
    Capability Tradeoffs

    With the background out of the way, What are the capability trade-offs of the different feature choices?

    Elevation Adjustment Methods

    Elevation specification can be done by external knobs (or direct mount adjustment, e.g. the
    Elcan), via reticle features, or a combination.

    Knobs: If the primary method of specifying elevation is by external knob, the knob will have
    "click" values. Each time the knob "clicks" to the next setting, the elevation setting will be
    changed by the click amount. Typical values of clicks are 1/4 MOA, 1/2 MOA, 1 MOA, or 0.1MIL.


    (Leupold 3.5-10x40mm M1 scope with 1/4 MOA-click external knobs, mounted on an Accuracy
    International (AI) AWP)

    Elevation Travel

    The scope's internal mechanical design and the scope mounts used determine the maximum range for
    which elevation can be specified. In the specifications for a scope, the maximum elevation
    travel is described as something like 60 MOA, 80 MOA, 100 MOA, etc. This is the total "top to
    bottom" travel of the erector assembly inside the tube.

    If the rifle and mounts are level, the elevation adjustment should be in the middle of its
    total travel when zeroed.
    For example, if we start with "0" at the bottom, a scope with 60 total MOA elevation will likely be zeroed at
    about 30 MOA up from bottom, and cranking it all the way up, it would stop at 60 MOA. In this
    case, the scope is limited to 40 MOA elevation from center/zero. This will limit the maximum
    engagement range by limiting the elevation setting that can be dialed. For example, if a certain
    308 load needs 31.5 MOA elevation for 1000 yards, the described scope will not be able to dial
    enough elevation. When it hits its maximum at 60 (30 above center/zero), it will still be 1.5
    MOA "short."

    The way to get around this is to use an inclined scope base. An inclined scope based has some
    downward "slope" built in. An inclined base with 20 MOA angle will shift the zero point in the
    scope further away from its top extent. For example, with the 60 MOA scope described before,
    instead of being zeroed around +30 MOA (its center), it would be zeroed at about 30 - 20 = 10 MOA
    up from bottom, and have about 30 + 20 = 50 MOA "up" elevation left. Now instead of running out
    of elevation travel trying to dial 31.5 MOA, the scope will dial freely up another 50 MOA-- when it is
    dialed to 31.5, it still has 18.5 MOA left for dialing to longer distances.

    Elevation Adjustment "Click" size

    The smallest elevation change possible using the scope's mechanism will in part determine the
    smallest target for which we can specify hold-over at an arbitrary distance.

    For example, if we have a scope with 1 MOA clicks, at 400 yards that will demarcate 4.2", so it
    will not be possible to dial the correct elevation to hit a 3" target at 400 yards with this
    setup. One adjustment setting might be just under the target, and the next would be 1" high over
    the top of the target.

    The tradeoff of fine clicks is that more of them are required to achieve the same elevation
    adjustment. For example, if 15 MOA are required to get to 600 yards, that would be 60 1/4-MOA
    clicks, but only 15 1-MOA clicks. The large, coarse click values can be faster to adjust in the
    field, at the expense of fine-grained adjustment ability.


    (Nightforce 3.5-15x50mm NPR2 with multi-turn, 10MOA, 0.25MOA click knobs, on an AI-AWM rifle. Photo by Frankie Icenogle used with permission.)


    If an external knob has a "zero stop" feature, the knob will physically stop turning at or near
    its "zero" setting. When the shooter wants to dial back down to his zero, he can turn it until
    it stops.

    A scope without a zero-stop, like the pictured Leupold, has a knob that will keep turning until
    the erector assembly bottoms out in the scope body tube. Each revolution the knob turns move the
    knob up or down, just like a jar lid. On a scope without a zero-stop, the shooter typically
    notes which "hash mark" the zero-revolution corresponds to.

    Single Turn, Two-Turn, and Multi-Turn Elevation Knobs

    In many scopes, a larger click size means fewer revolutions of the elevation knob are required to
    reach its maximum elevation. A good example of this is the Leupold M3 knob, which turns only one
    revolution but has 1 MOA clicks. The opposite example would be the Leupold M1 knob, which has
    3-5 revolutions depending on scope model and 1/4 MOA clicks.


    (Leupold MK4 M3 scope on a Remington 700. Photo by Frankie Icenogle used with permission.)

    Some scopes are designed to have very many small clicks in only one revolution. A good example
    of this would be the US Optics EREK knob, which has 90 clicks per revolution and can be ordered
    with 0.25, 0.5, or 0.1MIL click values, which would yield 22.5MOA, 45MOA, or 9.0MIL travel per

    Likewise, some scopes are designed to have just two turns of travel, with some indication to the
    user which revolution the knob is on. The best example is the Schmidt & Bender "Two Turn" PMII
    scope, which has approximately 27 mils of travel in two revolutions. Even the two-turn scopes have
    enough travel in the first revolution to shoot to 1000 yards with 308WIN.


    (Schmidt & Bender "Two Turn" PMII mounted on an AR10. Photo by Frankie Icenogle used with permission.)

    A single or two-turn scope simplifies elevation adjustment by freeing the shooter from keeping
    track of the current revolution of the knob. With a regular M1-style multi-turn knob, the
    shooter consults his log-book and reads 17MOA, then has to adjust his scope up one full turn
    (15MOA) and then two MOA past. With a single or two turn scope, he merely turns the knob about
    1/3 of one revolution until the markings for 17MOA are visible.

    Bullet Drop Compensators (BDC)

    Some scopes come with bullet-drop compensator (BDC) knobs. These knobs are calibrated for a
    certain load by having markings typically every 100 yards or meters on the knob itself, so the
    shooter can look for the distance on the knob instead of the angular elevation amount. If the
    shooter is engaging a target between marked distances, for example 450 yards, he will have to
    guess or look up in his data which click value between the 400 and 500 yard markings to use.

    A BDC knob is nothing more than a regular knob with markings that correspond to the load used.

    Tube Diameter and Mechanical Limit of Elevation Travel

    When the elevation knob is adjusted, it physically moves an assembly -- some lenses and the
    reticle -- inside the main tube body of the scope, just "under" the elevation knobs. This
    assembly is called the "erector" assembly because it inverts ("erects") the image coming from the
    objective lens. The erector assembly travels up and down as the elevation knob is turned, and
    left to right as the windage knob is turned. The movement of the erector assembly moves the
    "zero" of the reticle.

    The erector's movement within the scope body is limited by the side of the main tube diameter of
    the scope. Thus the larger the scope tube diameter, the more elevation travel will be
    mechanically possible. (It is also possible that the elevation knob mechanism itself limits
    travel before the mechanical limit of the erector. This is most common in "one turn" scopes like
    the Leupold M3.)

    Scope tube diameters include: 1" (25.4mm), 30mm, 34mm (Schmidt & Bender), 35mm (US Optics), and
    40mm. The advantages of the larger tube diameters are more elevation travel available and a
    stronger scope. The disadvantage of larger tube diameters is that the selection of scope rings
    is few, however, there are several high-quality ring sets available for 34 and 35mm tubes.


    (US Optics 3.2-17x44mm SN-3 with 35mm tube has approx 18 mils total elevation in two turns,
    ninety 0.1-mil clicks per revolution. Rifle is an AI-AWP.)

    Reticle Features

    The second method for elevation specification is to use reticle features. Many reticle designs
    have hash marks or dots down from the main cross hair which can be used for hold-over. In a FFP
    scope, the angles demarcated will be the same at any scope magnification. In a SFP scope, the
    angles demarcated will change as the scope magnification is changed. Thus, without overly
    complex calculations, reticle-based holdover is most useful in a FFP scope.

    Just like the "click" sizes, the spacing of the hash marks for reticle holdover in part determine
    the smallest engage-able target size. For example, if a reticle has 1 MIL demarcations (ie, in a
    mildot reticle) and you need to shoot a 10" square target at 600 yards, you need to hold
    approximately 3.4 mils high, so you'd put the target approx 40% of the way from the 3rd to the
    4th mark. If the target is small, there is no precise sight picture-- you're holding "in space"

    A more sophisticated reticle designed specifically for reticle-based holdover (and windage) is
    the Horus.


    (View through Horus H25 reticle at approx 12x magnification, targets at 100 yards)

    The Horus H25 reticle is mil-based, with small tick marks ever 0.2mil. A 308 shooter with the
    H25 reticle can shoot to 1000 yards using the reticle only.

    For example, at the TACPRO 2005 sniper match, there was a stage in which 5 targets had to be
    ranged and engaged with one shot each under a strict time limit. I ranged the targets with my
    laser and wrote their distances on my note-pad. As I moved from target to target, I only needed
    to look up the drop for that distance and use hold-over in the Horus H25 reticle. I didn't have
    to fiddle with any knobs. This demonstrates the speed advantage of reticle-based holdover. A
    shooter should try to memorize his drop values, and it also helps if he can remember the current target
    distances or have a spotter to communicate them.


    (Engaging multiple targets with the Horus reticle at TACPRO 2005. Photo by Frankie Icenogle used with permission.)

    Hybrid Knob & Reticle

    The last method for elevation specification is a hybrid, where the shooter might dial to an
    intermediate zero like 500 yards from his primary 100 yard zero, and then use reticle-based
    hold-under and hold-over for targets closer and further than the intermediate zero distance.

    Reticle and hybrid holdover has the advantage of being much faster than dialing elevation changes
    between shots at targets of different range. The downside is that sight picture precision is
    reduced because of the larger granularity of reticle features vs. typical knob click values.

    Again at the 2005 TACPRO sniper match, on a stage where I knew the distances beforehand (325,
    375, 500), I dialed to 375, and noted the hold-under for 325 (0.4mil), and the holdover
    (1.1mil). While shooting the stage, I merely used the appropriate hold-under/over points in the

    First Focal Plane vs Second Focal Plane

    In variable power scopes, a first-focal plane (FFP) reticle configuration means that the angular
    measure of the reticle features stays constant. No matter what magnification it is set at, 1 MOA
    will be 1 MOA and 1 MIL will demarcate 1 MIL.

    The FFP comes into play because with a wide range variable scope (my SN3 is 3.2-17x), dialing
    down the power will widen the field of view. Target to target transition times are drastically
    improved by widening the field of view. The ability to locate targets is enhanced by a wider
    field of view. To use reticle based holdover without the need to adjust to a specific
    magnification setting, the scope must have a FFP reticle.

    Another advantage of the FFP is that ranging and miss-spotting can be done at any power and yield
    direct accurate results.

    Exit pupil size numbers increase as the scope magnification is dialed down. That's the math
    behind the observation that a scope at a lower power will produce a brighter image than the same
    scope dialed up in power. During the day it doesn't make a difference. During the night, it makes
    a big difference in target ID and sight picture. For an illuminated reticle to be useful, its
    features need to demarcate the same at whatever magnification is needed for low light.

    A FFP reticle setup allows reticle-based and hybrid reticle/click holdover to be used at any
    magnification setting.

    There are some disadvantages to a FFP reticle in certain situations. As the magnification
    is increased, the width of the lines which comprise the reticle increase in apparent size and
    will obscure more of the target than the fine lines in a SFP reticle. Conversely, when the
    magnification is set near the bottom, for example at 4x on a 3.2-17x optic, the reticle lines
    "shrink" in size along with the target image and may become difficult or impossible to see in
    some lighting conditions due to their very fine width.

    Windage specification

    Windage works just the same as elevation. Knob clicks or reticle features can be used. The big
    difference is that the amount of wind hold off is much less than the maximum elevation required
    for the cartridge's maximum range. A typical 308 load might have 8 MOA deflection with a 20mph
    cross-wind at 800 yards, while it needs about 18 MOA of elevation at that distance. This means that
    windage travel is typically not an issue.

    Because wind changes can be very dynamic, using the reticle for windage hold-off can be more
    effective than dialing wind. For example, by the time you notice the wind and dial a correction,
    it may have changed already. Using reticle windage hold-off can be immediate.

    Lead specification

    Again, lead works basically the same as windage.

    MIL vs. MOA

    In principle, either system can be used. If you're thinking about or communicating elevation
    values (for example looking at data and then dialing or holding off), a typical elevation value in
    MOA for 308 looks like "11.25" which is four digits, but the same mil-based would be just "3.2" or
    two digits. (In fact you can go out to over 1000 yards before needing more than two digits of
    elevation in mils.) This is less information to process.

    Parallax Adjustment

    There are two types of parallax adjustment. The first is an adjustable objective, in which the
    objective bell itself rotates to adjust parallax. The second is a rotating knob typically on the
    left side of the scope, opposite the windage knob.

    The adjustable objective is optically simpler, meaning fewer lenses and more clarity and
    brightness, but the shooter must reach forward to the objective to adjust it. The rotating knob
    adjustment is more convenient since it's located closer, near the rest of the turrets, however,
    more lenses are involved which can reduce clarity and brightness.

    In either case, some parallax adjustment knobs or objectives are marked for range so the shooter
    can dial it based on the target distance. Others are not marked with distances, and it's up to
    the shooter to determine visually when the image is in focus and parallax-free.

    To determine if parallax exists at a certain distance, the shooter aims at an object at that
    distance, then moves his head slightly side to side and up and down without moving the rifle. If
    the reticle aiming point stays "on" the object, then it is parallax-free. If the reticle aiming
    point moves with regard to the object, then some parallax error is present.

    Reticle Illumination

    In some low-light conditions, it is difficult or impossible to see a black reticle on a dark
    target. Most tactical scopes are available with the option of an illuminated reticle.
    Mechanically, this consists of some type of external switch or brightness adjustment control, a
    battery, and a light source such as an LED (light emitting diode) inside the scope actually
    providing the light to the reticle.

    Some reticles are fully illuminated, but some reticles only illuminate their center portion. A
    fully lit reticle can be too "busy" visually, while a partially or center lit reticle might not
    illuminate all the reticle features. Brightness adjustment is critical. If the reticle is
    too dim, it might as well not be illuminated at all. If the reticle is too bright, it will wash
    out and obscure the target.

    There are several methods to turn on or adjust the brightness. Leupold scopes have an
    on/off/brightness turret at the 10:30'o'clock position on the ocular housing, just to the rear of
    the power adjustment ring. This is offset from the elevation adjustment knob, but still obscures
    it somewhat. Nightforce scopes have a simple on/off switch activated by pulling put the cap of
    the parallax adjustment knob. Schmidt & Bender have an auxiliary knob on the side for on/off and
    brightness adjustment. US Optics scopes with illumination similarly have a auxiliary knob
    somewhere on the turret housing of the scope, location depending on other scope features.

    Illuminated reticles, when turned on, are visible from the front of the weapon, through the
    objective lens as a red/orange light. The frontal visibility depends on the angle of
    observation, the intensity of the reticle, and scope design. If it is critical to not be
    observed from the target area, then reticle illumination must not be used.


    (During a night shoot, shooters are visible only by their cylume chamber flags.)

  3. Zak Smith

    Zak Smith Moderator Emeritus

    Dec 24, 2002
    Fort Collins, CO, USA.
    Brightness, Magnification, and Objective Size

    Most modern tactical scopes will have similar image brightness during the day, but differences at
    twilight and low or no-light can be dramatic. There are three main factors which affect low-light
    brightness: lens quality, magnification, and objective lens diameter.

    The easiest way to increase brightness is to dial down the magnification on adjustable scopes.
    There is an inverse relationship between magnification and image brightness. This is another good
    reason to choose an adjustable magnification scope.

    The second two factors affecting brightness are characteristics of the scope itself. Given two
    scopes with the same lens quality, the one with the larger objective lens will be brighter simply
    because it can focus more incoming light from the target area through the scope's lenses.
    Finally, lens and lens coating quality is critical to image brightness. Higher quality lenses and
    coatings will pass through more light and less brightness will be lost through the scope itself.

    There is a trade-off to be made between objective size and mechanical considerations. A scope
    with a 80mm objective will gather 4x more light than a 40mm objective, but it will be much heavier
    and will require extremely high mounts to clear the objective bell over the barrel. Mechanical
    considerations favor the smaller objective, and a lower sight over bore distance is preferable
    since it reduces the mechanical offset.


    (A US Optics SN-3 with a 58mm objective lens. Photo by Frankie Icenogle used with permission.)


    The following is the end-point I've arrived at after going through all of the above. A practical
    long-range rifle shooter who wants to shoot MOR, sniper, tactical, and field matches should pick a
    scope with the following features:

    1. Variable magnification in the 3-18x range. Low power is useful in low light, on close targets,
    and on movers. Higher magnification helps for target ID and sight picture at long range. Scope
    must have parallax and focus adjustment.

    2. Knob "clicks" no more coarse than 0.5 MOA. The standard clicks of 0.25 MOA or 0.1 MIL are great.
    0.1MIL is about 1/3 MOA. Clicks in this range are fine enough to allow precise specification of
    elevation for small targets.

    3. The elevation knob should have a zero-stop set up to allow either no clicks below "0" or up to a
    couple MOA "below" 0. The zero stop helps to prevent the shooter from being a full knob-turn
    revolution off from where he intends to be, and is easier to check settings in low light

    4. The reticle must be of a first focal plane configuration. The FFP reticle allows use of reticle
    features at any magnification setting, which is useful for target location, tracking of moving
    targets, fast engagements, spotting, and low-light.

    5. The reticle should have angular features in units useful for both hold-over/under and windage
    hold-off. Typical units would be 1/2 MOA hash marks, or 0.2 or 0.5 MIL hash marks. The Horus H25
    reticle appears busy, but is ideal for rapid engagements of multiple targets at different

    6. The angular units of the reticle features must match the angular units of the knobs' "click"
    values. There is no reason to have two different "systems" in use on the same scope. If the
    clicks are in MOA, the reticle features should be in MOA. If the reticle is in mils (e.g. Horus
    or Mil-dot), the knob clicks should be in mil units.

    7. Field-adjustable illuminated reticle. The illuminated reticle dramatically improves sight
    picture in some low light environments. The ability to adjust the brightness in the field is
    critical to prevent wash-out with a super bright reticle setting. The downside of an illuminated
    reticle is that it can indicate the presence of the shooter.

    8. Objective size. A good compromise point is a 44-50mm objective provided that the scope has very
    high quality lenses, such as those from Schmidt & Bender or US Optics. A larger objective size in
    a scope with lower quality lenses may be less bright than a smaller objective with high quality

    A Note About Cost

    Many people balk at spending $1000 or more on optics. This is misguided. High quality optics are
    one of the best places to spend money in a precision rifle system. Along with the rifle
    action, stock, and mounts, these costs are fixed over the life of the rifle. The cost of training,
    ammunition, and barrels dramatically eclipses those fixed costs.

    To illustrate the point, let's analyze the cost of training with a high-end factory precision rifle
    (AI-AE) using a top of the line S&B or US Optics scope for 5 years. A rifleman with a moderate but
    regular training schedule might shoot 3000 per year. If he is shooting 308, a realistic barrel life
    might be 5000 rounds until the groups increase beyond his spec. Over the 5 years, that will be
    15,000 rounds and 3 barrels. For ammunition cost, we will use a conservative cost from what
    reloaded ammunition might cost.

    Rifle cost         $2500 (AI-AE minus the first barrel)
    Scope cost         $2100
    FIXED COSTS -----> $4600
    3 barrels          $1800
    15,000 reloads     $6000
    CONSUMABLE COST -> $7800
    This comparison doesn't even include the cost of formal training, match fees and travel costs. If
    you plan on shooting regularly to achieve a superior level of proficiency, it makes sense
    to buy the best rifle and scope you possibly can.


    Based on the above list, there are basically three choices that meet all of them:

    1. US Optics SN-3 3.2-17mm

    2. Schmidt & Bender PMII

    3. (Caveat) Leupold Mark 4 "FF". The M1 version of this scope has no zero stop. The M3 version of
    this scope has a zero stop, but coarse 1 MOA clicks.

    Good Shooting & Stay Safe.


    $Author: zak $
    $Date: 2005/09/01 03:43:54 $
    $Revision: 1.18 $
  4. ocabj

    ocabj Member

    Jan 22, 2003
    Riverside, CA
    That's a very extensive write-up. Good photos, too.

    I don't know why Leupold hasn't made an 'M2' scope. Something between the M1 and M3 with .5 MOA per click knobs, as opposed to the 1/4 and 1 MOA clicks of the M1 and M3, respectively.
  5. Zak Smith

    Zak Smith Moderator Emeritus

    Dec 24, 2002
    Fort Collins, CO, USA.
    The new Mark 4 1.5-5x20mm MR/T M2 has "M2" knobs. :neener:
  6. SpookyPistolero

    SpookyPistolero Member

    Jun 26, 2004
    Central Kentucky
    Ah, I think I went cross-eyed!

    Just kidding, that was a very informative and helpful review, especially for someone like me who doesn't know squat about long range riflery.

    The last picture was really cool, nice endcap.
  7. WayneConrad

    WayneConrad Member

    Feb 21, 2005
    Phoenix, AZ
    Great article! Thanks!
  8. Joe Mamma

    Joe Mamma Member

    Mar 3, 2003
    Excellent. That was the best article on this subject that i have ever read.

    Joe Mamma
  9. Maddock

    Maddock Member

    Dec 25, 2002
    Very impressive article.
    Thank you.
  10. swingset

    swingset Member

    Dec 25, 2002
    Very informative, great write up & pics.
  11. Zak Smith

    Zak Smith Moderator Emeritus

    Dec 24, 2002
    Fort Collins, CO, USA.
    Gonna give this a bump...
  12. Gordon

    Gordon Member

    Dec 26, 2002
    central Kali.
    Holy cow ! What a difference 20 years makes! When I was shooting Practical rifle from 83-88 in California, it was much different under IPSC auspices. Here we ran to various positions and fired from various mandated positions. When I started I, along with many others, used an HK-91. The M-1a shooters were there too in ever increasing numbers. Just a few bolt guns in 'deer rifle' configuration.The AR-15 shooters had a problem as 55 grain bullets (no heavy ones yet!) didn't do well at 300 and past.
    Anyway I believe it was 85 or so when I got the brilliant idea to switch over to my Brown Precision Urban Sniper rifle, newly out. I have one in .308 and later in .223 1-9" . The .308 Model 700 Rem one was modified for m-14 mags, and printed SUB MOA with Lake City Ball with it's 8x56 Kahles Super S Helia on Buehler Mounts (26mm). This with a long Harris bipod allowed quick rested kneeling shots and every body yelled CHEAT:fire: - to NO avail!:neener:
    The next year , after I WON the western regional everybody seemed to switch to a bolt gun and I was 'over the hill' not wishing to get into the 'race' , I just wanted to develop "skills":cool:
  13. Zak Smith

    Zak Smith Moderator Emeritus

    Dec 24, 2002
    Fort Collins, CO, USA.

    There are still the "IPSC-style" action-shooting rifle matches. For example, we have a local match which has 3 or 4 3Gun-style stages, but it's all rifle and we shoot out to 400 yards, run-n-gun, on the clock. Then most of the larger 3Gun matches have rifle stages out to about 350. I consider this to be basically "medium range."

    This optics paper really is more for the sniper-style precision / long-range stuff.

    best regards
  14. NMshooter

    NMshooter Member

    Jun 13, 2004
    I thought this thread should have been tacked the first time around, or at least put somewhere where we can find it.

    Big Hint to any moderators reading this...;)

    Oh, and Zak, I appreciate you taking the time to make this post.
  15. rockstar.esq

    rockstar.esq Member

    Dec 9, 2004
    Zak, great post! I know that you are writing more to the sniper/ tactical etc. crowd but I was wondering what you make of the Shepherd scopes? They seem to fall into your listed specifications quite well. As a plus for a newbie, they appear to be more instinctive to use.

    Here's the link
  16. Gewehr98

    Gewehr98 Member

    Dec 24, 2002
    A change in preferences, I guess.

    Used to be First Focal Plane reticles were considered a Bad Thing®, because the reticle grew in size as one increased magnification in their variable power scopes, making for a very busy field of view at higher magnification. I didn't think it was that much of a problem to remember that a given magnification, say 10x on one of my mil-dot scopes, was the correct one for accurate ranging and holdover. There are many high-quality scopes that were eliminated from Zak's considerations simply because of the First Focal Plane thing.

    Likewise, many quality optics were eliminated from his consideration because they didn't have zero-stops on their turrets. I consider that to be a not-issue, because many long-range shooters, including myself, do exactly like Zak stated:
    Every new scope I get gets checked out, and the elevation and windage traverses get checked click by click against the turret markings for mechanical zero. I then either mark or reindex the turret handle for baseline zero, depending on the cartridge ballistics and scope. I find that important, especially when using bases with 20MOA of elevation cant.
  17. Zak Smith

    Zak Smith Moderator Emeritus

    Dec 24, 2002
    Fort Collins, CO, USA.

    I can't speak to why/if they used to be considered "Bad", but I can address the point made here. A FFP reticle always demarcates, by its line width and features, the same angular measure. It doesn't exactly become "more busy" as the magnification is increased, it's just that the features don't shrink in angular demarcation like they do on a SFP reticle setup. One could argue that a FFP reticle actually becomes less busy as power is increased since parts of the reticle will disappear from view. I don't think a reticle as simple as a mildot is really "busy" in either case.

    The other issue with FFP reticles is that at maximum magnification, their line widths are generally thicker than the line widths of a SFP reticle at the same power. For example, compare a USO or S&B mil-dot at 15x to a Nightforce NPR2 reticle and NPR2 will appear to have thinner lines. This does set a limit on the minimum size target (ie, a red dot on a white background) on which one can get a sight picture.

    I can obtain a sight picture with a FFP Horus H25 reticle on 1/2" dots (just), and easily on 1" dots. So in the absence of any other larger target-ID features (ie, a single black dot on a white background), a FFP is sufficient for 1/2 MOA (limit) and easily 1 MOA targets. I have done the same shooting 7" plates at approx 675 yards (0.99 MOA). So I guess I don't see that as a big problem with FFP scopes.

    The article is primarily about Practical precision / long range rifle shooting. IMO, this is distinguished from regular long range target shooting by: having to locate and ID targets, shooting multiple targets at different headings/distances, shooting single and multiple moving targets, shooting from improvised or sub-ideal positions, and then doing it all communicating with a partner/spotter, at night, and under time pressure.

    Variable magnification scopes help in basically two ways over fixed power scopes:

    1. Larger field of view at lower magnification. This helps dramatically when trying to locate targets (on your own, or when your spotter is saying "5 mils left and 1 down from the green bush"), when you will be engaging multiple targets in series (can keep more of them in view) or need to observe multiple things spaced apart, or when engaging moving targets. It's pretty common to see guys with their scopes set at maximum power, or fixed 20x+ scopes, have a hard time "finding" a target, because they get so little context in the scope's field of view.

    2. Larger exit pupil and a brighter image. If shooting in low light, turning down the magnification will provide a brighter image and usually makes the difference between being able to make out the target and just seeing vague shapes in the dark.

    (Also, mirage is often less apparent at lower magnifications. Sometimes you want to see it to judge wind, but at other times there is enough to obscure the target sight picture-- e.g. shooting 1 mile over burning debris.)

    Given these, if you are going to use reticle features for spotting (and commicating) or in aiming, it makes sense to have a FFP reticle. If you are going to do so, but use a SFP reticle, you'll have to train for a second method of doing all those things when you cannot dial to the max power.

    Everyone without a zero stop trying to verify their zero setting at night I've seen, has had to resort to breaking out their Surefire to visually check. With a zero-stop, you can tactilely determine the elevation is zeroed. Furthermore, I have seen enough people make the mistake in the field, when NOT under time pressure, and in daylight, that I believe it's important.

    Given the state of practical long range / precision shooting, I tried to look at the capabilities which provide the best tool-set to solve the problems. A skilled shooter can "get by" with a SFP duplex reticle, but that doesn't mean it's the best choice.

    I am not trying to say that a scope which doesn't meet my "criteria" is junk. There are many quality scopes out there which are great for some uses, but not for others. If someone shoots a different "game" with different problems to solve, they will likely come to different conclusions. For exmaple, if I were shooting primarily K.D. for groups, I would get a fixed power or variable SFP with the finest reticle I could get.


    I haven't played with a Shepherd scope, and I couldn't really get a lot of info from their web-site. Just from the pictures there, one concern I have is that I couldn't see any regular, calibrated windage references in the reticle. I believe the best "budget" choice for someone getting into the practical rifle stuff is the Leupold 3.5-10x40mm "FF" M1, mildot. Hunting is a whole different topic...

    I appreciate the comments and discussion! This is good.

    best regards
  18. Dionysusigma

    Dionysusigma Member

    Mar 27, 2003
    Okay City
    Nice write-up. Good to know in case I mysteriously stumble across $8,000 sitting in the bushes.

    'Til then, I'll stick with my AK.
  19. Gewehr98

    Gewehr98 Member

    Dec 24, 2002
    Now, now...

    Let's not forget, "the difference between men and boys". :D
  20. CB900F

    CB900F Member

    Feb 22, 2003

    Damn! I forgot & sent my permission by first class instead of express. I'm sorry.

    :D 900F
  21. dakotasin

    dakotasin Member

    Dec 26, 2002
    or until you want to start hitting targets... and then the targets get smaller and farther... ak's are fun to blast away at 25 or 50 yards, but for real fun, send a bullet whistling over the landscape to a target the size of a 20 ounce pepsi bottle at 800 yards and beyond... that's fun!
  22. Maylene

    Maylene Member

    Dec 16, 2005
    Haha amen :evil:
  23. RugerOldArmy

    RugerOldArmy Member

    Sep 18, 2004
    Earth, Milky Way
    Very nice writeup Zak.

    It is appreciated.
  24. bogie

    bogie Member

    Jan 2, 2003
    St. Louis, in the Don't Show Me state

    One thing you may consider looking at in the future is kinetic energy effect on scope tube - big tube/mirage shades, large optics hanging out there, etc., bring a multitude of Newton's bugaboos out.

    Also, the reliability of the mechanism needs to be triple-checked. Some benchrest shooters are leaning back toward fixed scopes with external adjustments (i.e., like John Unertliy's stuff, only updated). Some crosshairs, even on $8-900 scopes, can move as much as a quarter MOA under recoil. Not a good thing. Charlie Hood makes a dealie that can be used to mount two scopes in a stout recoil rifle to make sure that both are saying the same thing... If one's different, then you get to figure out which one it is... But that's generally fairly easy.
  25. bogie

    bogie Member

    Jan 2, 2003
    St. Louis, in the Don't Show Me state
    Accuracy costs money. This is the equivalent of a top-fuel dragster. The AK is the equivalent of a 1960s Volkswagen. They'll both get you somewhere, but they've got vastly different uses.

    Engaging a whole buncha targets inside 100 yards? AK's a good choice (but a better choice might be shotguns with 00 Buck...). One shot at 500 or more yards? I'll go for either my 6BR custom or my Savage .308... At a thousand yards? Sorry, but with my existing rifles, I don't really feel confident in that direction. My Savage .308 _might_ make it.

    I would disagree with one thing in the original manuscript.

    3 barrels for 15,000 rounds? Are you serious? I'd figure at least a dozen. Go through 'em, see which are good shooters, and which are passable, and use the good ones for matches/other accuracy concerns, and use the passable ones for practice. The barrel wrench and vise will only cost $100 or so, and will rapidly become an indispensible part of your toolbox.
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