.308 Trajectory and Ballistics Questions


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mountain_cowboy
October 13, 2004, 11:09 AM
Forgive me for these elementary ballistics questions. I'm used to shooting single action revolvers at 25 yards. Okay, I'm slowly trying to learn about the ballistics of my favorite calibers. So far, I've just sighted my rifles in at 100 yards with factory ammo and gone from there, with no ballistics data. Now, I'm trying to read up on bullet drop, trajectory, and different loads. However, I don't get the bullet drop differences for zeroing at different yardages. For example, looking at the charts here for Fed GMM:

http://www.snipercentral.com/308.htm

When zeroing a .308 rifle at 600 yards, the trajectory of the bullet peaks around 300 yards, right? Meaning that when the bullet is fired, its path rises until it reaches 300yards, where it's roughly 35" above where it was originally aimed. Then the bullet starts to drop, and at 600 yards, it is in line with where it was aimed. It then continues to drop increasingly rapidly at it moves out. Okay, that makes sense to me.

Now, if you look at the lower chart for a 100 yard zero, out at 200 and 300 yards the bullet has already dropped considerably. Why wouldn't the bullet's flight arc still be on the rise?

Does anyone have a link to a diagram with maybe some arcs drawn on it showing bullet path for zeroing at different yardages? Thanks.

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mountain_cowboy
October 13, 2004, 11:19 AM
Okay, I may have already answered my own question. I took some graph paper and layed out equal sections representing 100 yard increments out to 600 yards. Then I drew a pretty symetrical arc from 0 to 600 yards, peaking the the 300 yard mark and zeroing at 600. I then traced that arc on another sheet of paper and transfered it down to where it began at 0 yards and zeroed in at 100. I continued the arc on down, and can see that at 200 and 300 yards the path would be much lower relative to the ground than at 100 yards. So, I guess it's all about the angle of the barrel and where you're aiming, right? Adjusting the scope/sight for zero is just raising or lowering angle of the barrel relative to the ground. Technically, zeroed for 100 or 600 yards, the flight path and the arc are still the same irrespective of the ground's horizontal. You're just changing the angle at which they start> Right? Does that make sense, or am I totally off?

SLCDave
October 13, 2004, 11:26 AM
Right! If you are sighted in for a 600 yard shot, your sights will be higher in the rear, causing you to angle your barrel up for the longer shot, which compensates for the drop of the projectile.

Al Thompson
October 13, 2004, 11:45 AM
MC, you've got it. I'd also point out that paper ballistics and "actuual" ballistics don't always match. You have to shoot your combo (ammo, rifle, scope) to see how it works and where.

Bwana John
October 13, 2004, 12:04 PM
Physics. It actually is rocket science.
If Ax^2 + Bx + C = 0 then x=(-B +/- (B^2-4AC)^-1)/ 2A,
and y=-(x-h)^2 + k
and R= (V^2 sin (2)theata)/g
and V(y)= (mag) sin theata, V(x)= (mag) cos theata
and B.C., temp, pressure, ect...

Seriously though, you got it, the more you elevate the barrel between 0-~33 degrees the farther the projectile will travel horizonaly, and the higher the parabolic arc.

mete
October 13, 2004, 12:56 PM
Something that is confusing to many is that the data is often 'line of sight' rather than from the bore . But line of sight can be 1" or more depending on iron sights or scope. The paper numbers are only a guide check it out with your gun.

30Cal
October 13, 2004, 01:11 PM
Checking the glossary is always good before spending too much time looking at the tables:

Bullet drop is the vertical distance between the bullet and the line that runs down the bore. It starts at zero at the muzzle and becomes more negative from there on out.

Bullet path is the vertical distance between the bullet and the line of sight (line between your eyeball, through the sights and to the point of aim).

FWIW, for nominal .308 and .30-06 loads in 147FMJBT, 150FMJ, 168HPBT, 173FMJBT, 175HPBT, the standard come-ups are:
from 100 to 200 yards: 2 MoA
From 200 to 300yards: 3 MoA
From 300 to 600yards: 12-14 MoA

These are for iron sights, but even with a scope, they should still get you on the paper.

Art Eatman
October 13, 2004, 04:05 PM
For the general range of deer cartridges, from the .257 Roberts on up to the '06, you can basically figure for a scoped rifle:

Zeroed at 25 yards will get you near the center of the target at 100 yards.

Two inches high at 100 yards will be about dead on at 200 yards, and about six inches low at 300 yards.

Since most deer are shot at within 250 yards--and, typically, closer--this sight-in for zero at 200 works quite well.

The .243 and the .25-'06 will be a bit flatter, as will stuff like the .264, 7mm or .300 maggies. The .30-30 will be "droopier".

FWIW, Art

Zak Smith
October 13, 2004, 06:17 PM
You can't accurately predict trajectory without using numerical methods on differential equations. This is why everybody uses ballistic calculators based on an appropriate drag model to general drop tables, and then cross-checks them with real shooting. An accurate model has to take into account atmospheric conditions, etc.

-z

308win
October 13, 2004, 06:29 PM
I think you already have your answer. At the risk of telling you something you already know. If you were to drop a bullet from the same height as the end of your barrel at the same time that a fired bullet exited the barrel, the two would hit the ground at the same time. Thus, the more acute the angle of your barrel, the farther the bullet will travel before it hits the ground; velocity, bullet weight, shape, BC, etc. have nothing to do with it. May not sound intuitive but that's the way gravity works.

Zak Smith
October 13, 2004, 06:33 PM
Thus, the more acute the angle of your barrel, the farther the bullet will travel before it hits the ground; velocity, bullet weight, shape, BC, etc. have nothing to do with it. May not sound intuitive but that's the way gravity works.
I don't agree w.r.t velocity & BC.

If the angle between the bore and the horizontal is A, then sin(A) is the initial vertical velocity. The higher the initial vertical velocity is, the longer the bullet will be airborne... just like if you throw a ball up at 100m/s vs. 1m/s. BC comes into play because it additionally retards the vertical velocity.

-z

schromf
October 13, 2004, 06:51 PM
Two inches high at 100 yards will be about dead on at 200 yards, and about six inches low at 300 yards.

Agree: the KISS version for most modern cartridges is 2" high @ 100 yds.

Be aware certain balloon trajectory cartridges, don't fit the above rule, 30-30, 45-70, 7.62X39, and 38-55 you need to figure out the bullet path for. Not a all inclusive list just an example of cartridge types you need to calculate.

How do you do that easily? Follow this link:

http://www.norma.cc/sida/eng/index.html

Highlight: Ballistic, Ballistic, and it will bring up a very useful ballistic calculator. You can use define your own bullet, add the values: Bullet weight, velocity, ballistic coeficient, and you can play around around all you want. If you have some time to waste, compare various cartridges and loads, especially at 200 and 300 yards. Most common cartridges are similar, it takes something special to get outside a range at that distances. Large bores at slow velocities definately want to drop, and .264 through .308 calibers at medium velocities group very similarly. It takes either a very good BC or a lot of velocity to change it much.

Dave R
October 13, 2004, 06:58 PM
Then I drew a pretty symetrical arc

There's your problem. The ballistic arc is not symmetrical. Its a parabola. As the bullet moves past 200 yards, it is slowing faster and faster. That means if moves down more for each foot it travels.

I think the rule of thumb is the drop doubles for each additional 100 yards?

So if you're 6" low at 300 yards, you'll be 12 more (or 18" total) low at 400 yards. And 24 + 18 or 42" low at 500 yards.

Someone with a ballistic table, am I somewhat close here?

Zak Smith
October 13, 2004, 07:08 PM
_BC_ _MV_ 0 100 200 300 400 500 600 700 800 | YARDS
0.447 2800 > -1.76 -0.00 -3.38 -12.88 -29.35 -54.45 -89.18 -136.28 -197.05 | > 308 168gr SMK
2798 2595 2402 2216 2040 1871 1713 1564 1430 | fps, velocity
Far zero at 99.5, maximum 0.04 at 87.5, falling -0.70"/yd, angle 0.00115 rad

Art Eatman
October 14, 2004, 01:00 AM
DaveR, for the .308/'06 sort of cartridges, you can figure that if zeroed at 200 you're gonna be about two feet low at 400 and about four feet low at 500. About 6.5 to 7 feet at 600, and Mr. Sierra sez around 30 feet low at 1,000 yards.

I've found the Sierra reloading handbook to be the lazy man's friend when it comes to figuring trajectories...

:), Art

Dave R
October 14, 2004, 01:24 AM
Thanks, Art and Zak.

bcochran
October 14, 2004, 01:42 AM
1. You buy the software from Sierra Bullets
2. You play with the parameters until you get what you want
3. You print out the results that you desire
4. You laminate the printed results and attach two to the rifle and take them to the range.

And to think that I could skip Geometry, Trigonometery, Calcculus and other mathematical disciplines in this day and age!

schromf
October 14, 2004, 01:52 AM
1. You buy the software from Sierra Bullets

Or you can go to Norma site I listed above and get it for free. See my previous post in this thread.

Art Eatman
October 14, 2004, 10:51 AM
schromf, you're right. However, my system works even when Terlingua Creek eats powerline poles and phone lines and the computer doesn't work cause there's no 'lectrumicity or phone...

Besides, my daddy and my uncle learned all that stuff back some 80 years ago and, later on, told me, and that was before Norma or Sierra or Internet. Human memory can be as useful as computer memory, neh?

:D, Art

twoblink
October 14, 2004, 11:21 AM
Maybe it's just me, I always did the "If it's on at 25 yards, it's on at 125 yards" thing and called it a day as far as my 308 was concerned..

zeroing at 25 just seemed easier.. and for the most part, it was always on at 125 as predicted..

yzguy
October 15, 2004, 04:03 PM
BTW this link pops up the calculater thing directly: :)
http://www.norma.cc/htm_files/javapagee.asp

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