Shooting Exercise I-A and I-B


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echo3mike
May 16, 2003, 10:35 AM
Shooting Exercise I-A: Mirage

-There are a number of ways to determine wind speed and direction. At longer ranges, one of the most predictable is mirage. It's often difficult to see the leaves rustling from a 3-5mph wind when you're 800yds away. However, by focusing the spotting scope to rougly 1/2 to 2/3 of the distance to the target, the wind speed and direction can be obtained...with practice and experience.

Alot of reading mirage could be called SWAGging, but there are some basic guidelines to this "art". A "No Wind" value means just that...there's no wind blowing at the point of focus, and the mirage is generally going straight up. This can often be confused with a "boil", which is usually seen when you're looking directly into or away from the direction the wind is blowing, i.e. headwinds or tailwinds. A rough idea of wind direction is usually indicated by which way the mirage is "moving"...left to right / right to left. Wind speed can be guesstimated by how fast the mirage "looks"...if it's a stronger wind, the mirage will move faster, and the moving mirage might look closer together.

Exercise I-A
-Using the data obtained in Shooting Exercise I (http://www.thehighroad.org/showthread.php?s=&threadid=22731) (your data / my data...it doesn't matter) and ANY OTHER SOURCES,
A) Where would you point your spotting scope to obtain the wind direction?
B) What would the mirage look like when the spotting scope is pointed toward the target (speed and direction)?
C) You're shooting into a "rapid boil". Would there be any changes to your POI (theoretically)?




Shooting Exercise I-B: Meteorlogical and Environmental Effects

Most of us don't shoot in vastly different locations with any frequency. The effects of weather and the environment don't play a signifcant role in our ranging or sight adjustments. But the effects of environmental and meteorlogical (ENV/MET) variables can have a drastic effect on POI when the location changes dramatically. Most of the ballistic tables are prepared around Standard Atmospheric Conditions (SAC): 59.5 degrees F, Barometric Pressure of 29.92in Hg, 0% Humidity, and 0ft ASL. At short range (100 - 600yds), the effects of different ENV/MET variables aren't that important and can be neglected. And, depending on the caliber, even at ranges out to 1000yds the change isn't that great. But it can add up and change your POI considerably.

As with mirage, there aren't any hard, fast rules but there are guidelines: Temp up / sights down...Humidity up / sights down...Baro P up / sights up.

Exercise I-B
-Using the data from Shooting Exercise I (http://www.thehighroad.org/showthread.php?s=&threadid=22731) (mine/yours..irrelevant: Sierra .308cal 150gr FMJBT, BC of 0.226, MV of 2740fps) and ANY OTHER SOURCES, what are
A) The elevation for 800, 900, 1000 yards at SAC
B) ..at 95 degrees
C) ...at 85% humidity
D)...at 30.35 in Hg
E) all of the above combined
F) Why, when you correct for Barometric Pressure, you simultaneously correct for altitude?

S.

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Steve Smith
May 16, 2003, 10:57 AM
Pulling my response because I don't want folks to think I'm trying to be a know it all. I like playing too. :(

Newt
May 16, 2003, 03:04 PM
If the folks here are anything like me, then they want to see your response. Myself, I would love to know/learn how to figure all of this stuff out. It fascinates me. I don't think anyone would have the patience to sit down and teach me, though.

Newt

echo3mike
May 17, 2003, 11:02 AM
What the heck Steve...light it up. I'm thinking about doing a couple more exercises followed by a full mission profile.


Newt: Follow along and work the problems. Mirage is probably best learned by experience but the basics are good to know. The environmental stuff can be found by going to one of the ballistics freeware sites and playing with the input variables. Not as good as getting out and shooting, but it'll give you an idea of what's involved.

S.

hps1
May 17, 2003, 02:32 PM
Exercise I-A
-Using the data obtained in Shooting Exercise I (your data / my data...it doesn't matter) and ANY OTHER SOURCES,
A) Where would you point your spotting scope to obtain the wind direction?
B) What would the mirage look like when the spotting scope is pointed toward the target (speed and direction)?
C) You're shooting into a "rapid boil". Would there be any changes to your POI (theoretically)?

A) Into the boil would mean you are looking into or with the wind.

B) Mirage appears as heat waves. The long wave, slow moving heat wave is worth less correction than a short, fast moving wave. Easiest way to remember how mirage affects POI is to remember that the immage of your target is carried along with the mirage so that the target is not where it appears to be. Sight corrections are made into the mirage.
C) Sight correction would be down in a boiling mirage as the target is actually lower than it appears through the boil.

Exercise I-B
-Using the data from Shooting Exercise I (mine/yours..irrelevant: Sierra .308cal 150gr FMJBT, BC of 0.226, MV of 2740fps) and ANY OTHER SOURCES, what are
A) The elevation for 800, 900, 1000 yards at SAC
B) ..at 95 degrees
C) ...at 85% humidity
D)...at 30.35 inH
E) all of the above combined
F) Why, when you correct for Barometric Pressure, you simultaneously correct for altitude?

81 mm mortar experience came in handy with the trajectory of the 150 gr. bullet @ 1000 yds. :D All values added to 100 yd zero.

A) 800=+364" ;900=+515";1000=+703"
B) 800=+342"; 900=+485";1000=+662"
F) Barometric pressure drops with increase in altitude.

Having done most of my shooting below 125' of elevation and since my ballistic calculator does not cover C & D, will defer to someone else on these. I would venture a guess that the higher elevation/lower BP, the less bullet drop you would have? Someone enlighten me................Steve, you live up near the clouds!;)

Regards,
hps

echo3mike
May 17, 2003, 10:01 PM
Air density tends to decrease with the following:
-BP drop below SAC;
-Air temp rises above SAC
-High temps + high humidity
-High Altitude.

Decreased air density means less drag on the bullet because the air is thinner. Air density is more a function of barometric pressure than it is of altitude. All the variables associated with shooting at a higher elevation are correctable by adjusting for the BP. For instance, I live at about 400' ASL. Take a look here (http://www.digitaldutch.com/atmoscalc/), and you'll see that the Standard Conditions at Altitude is 29.49 in Hg. Right now, the BP is Silver Spring, MD is around 30.15 in Hg. If I were to make sight adjustments based on Standard Conditions at Altitude, I'd be hitting the target lower at long range, close to .5 MOA.

As another example, in the exercise the difference at 1000yds from the SAC (29.92 in Hg) elevation to the BP adjusted (30.35 in Hg) is a difference of about 16" higher than for the SAC elvation. For question E, the amount of change is pretty severe.

BTW, great explainations of mirage. Still going to have differences in the exact elevations on the ENV / MET stuff but it looks like your percentage of increase / decrease is pretty close. I'll wait to post...I wanna see what Steve says, too!

S.

echo3mike
May 18, 2003, 08:17 PM
Exercise I-B:

A) SAC elevation for
-800yds: 48.3MOA / 404.5"
-900yds: 62.7MOA / 591.2"
-1000yds: 79.2MOA / 829.4"

B) At 95 degrees F
-800yds: 44.8MOA / 375.5"
-900yds: 579MOA / 545.8"
-1000yds: 72.8MOA / 762.1"

C) At 85% Humidity
- 800:47.4MOA / 397.0"
-900: 61.6MOA / 580.2"
-1000: 77.7 MOA / 813.9"

D) At 30.35 in Hg
-800: 49.1MOA / 411.2"
-900: 63.8MOA / 601.1"
-1000: 80.5MOA / 843.3"

E) All of the above combined:
-800: 43.0MOA / 360"
-900: 55.5MOA / 523.5"
-1000: 69.8 / 730.8"

It's pretty easy to see that the environment can have a serious impact on bullet drop at longer ranges. Even close in the effect can be noticable, like having the zero change in July for ammo you last shot in November. As you can see in question E, that the total cumulative effect is almost 10 MOA or 100 inches less than the come up at SAC.


I used the JBM page again. It's quick and proves the point. I also used Sierra's 4th Ed Rifle Reloading Manual for parts of the description for question F.

More follows,
S.

Quintin Likely
May 18, 2003, 10:35 PM
My head hurts... :uhoh:

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