confused on ballistics


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M_Olson
October 15, 2008, 08:25 PM
Disclaimer: This post is pretty long-winded and I may have misused some terminology so please feel free to correct anything or everything I say.

I am officially, thoroughly confused. I think it’s from trying to get a bunch of information from people who act like they know what they are talking about...key word being "act". My first rifle shooting experience, outside of plinking around with a .22 at about ~25 yds or so, began when I bought a Mosin Nagant 91/30.

I brought it to the range to try it out and found that on the lowest setting for the rear sight, it shoots about 3-4" above point of aim at 25 yds, 8-10" high at 50 yds, and about 14-17" high at 100 yds. After doing a bit of research, I found out this was fairly normal for the 91/30's as they are zeroed at about 200 meters and can shoot high without the bayonet on. Before doing this research, I was told by someone, who shall remain unidentified, that it didn't make sense that the rifle was doing that as the bullet should fall below point of aim before 100 yds. This made no sense to me, but he talked like he knew what he was saying. This is where my confusion began.

Just last weekend I went with the same person on an early antlerless firearms hunt. I borrowed my boss’ BAR 30.06 and brought it to the range to make sure it was sighted in properly. I started at 50 yds, where it shot about 3” high. Being told by the certain person that it would shoot higher at 50 than it would at 100, I figured it would be about dead on when I got to the 100 yd. Well, I got to the 100 and it was shooting about 6” high of center. So I zeroed it at 100 and knew that at 50 it would be about 3” low. When I told the gentleman this, he said it sounded wrong again.

Is there something wrong with me? When I picture the process of the bullet following a trajectory, this makes perfect sense to me. Does it make sense to be shooting 2” high at 50 yds and 1” low at 100 as he says he does? Also, if anyone has a site that does a good job of explaining rifle ballistics to a beginner? I'm getting tired of asking people questions that I think they might know, only to find out they don't know what they're talking about or I found an unreliable source in my research. I truly don't know what's what.

Thanks for taking the time to read this horribly long winded post.

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M_Olson
October 15, 2008, 10:24 PM
bump for a frustrated beginner

Tony Williams
October 15, 2008, 10:33 PM
Your understanding is correct. You have to imagine two different lines extending from your gun: a straight line for the line of sight, and a curved line to indicate the trajectory of the bullet.

Since rifle sights are mounted a few inches above the barrel, the sight line will be above the trajectory at short range. The bullet then climbs until it crosses the sight line (at which distance your sights are dead-on). It then carries on climbing until it reaches the apex of its trajectory, at which point it reaches the maximum height above the sight line. From then it begins to fall as gravity takes effect, until it crosses the sight line again (the second distance at which your sights will be dead-on) and from then on the hitting point will drop further and further below the point of aim.

The actual distances at which the sights are dead-on wll depend on the trajectory of the bullet, the range at which the sights are set, and the height of the sights above the barrel.

The fact that there are two points at which the sights are dead-on can allow a shooter who knows what these are to zero his sights at the shorter range, knowing it will then also be on target at the longer range.

Having said that, for a powerful high-velocity cartridge, the second dead-on point may be beyond the effective hunting range of the rifle.

There's a basic introduction to ballistics on my website HERE (http://www.quarry.nildram.co.uk/ballistics.htm), although it doesn't answer your specific question. However, if you want to work out the details for yourself, you can try a site like this: http://www.eskimo.com/~jbm/calculations/calculations.html

M_Olson
October 15, 2008, 10:52 PM
Thank you for your response Tony.

So, shooting the 30.06 and hitting low at 50 yards and dead on at 100, would it be safe for me to assume that the bullet is impacting the target at its first point of contact with my sight line?

gvnwst
October 15, 2008, 10:53 PM
probably. the '06 dosen't drop THAT much... :D

M_Olson
October 15, 2008, 11:14 PM
That's what I figured. This guy has my head all screwed up, telling me that 30.06 crosses and drops below line of sight within 100 yds. I couldnt see how it could drop that fast, but if I hear something said enough times with enough conviction I tend to doubt my own conclusions.

skinewmexico
October 15, 2008, 11:15 PM
I'd be shocked if a 3006 was lower at 100 than 50. Maybe if you're shooting a bow, or black powder. Look up an online ballistic calculator, or Remington's, and see what they show. If you're an 1-1/2" high @ 100, you should be close to dead on @ 200.

Ringtail
October 15, 2008, 11:50 PM
The best way to learn is to shoot. Don't take anyone's word for anythig. If you have questions about your rifle's trajectory, take it out and shoot it at various ranges and measure you bullet's rise or fall. That will make you the balistic expert and a better shooter as well. Not to mention that more shooting is lots of fun.

dmazur
October 16, 2008, 12:07 AM
Here's a fairly plain explanation that may help -

http://members.aye.net/~bspen/trajectories.html

The 3" high / 6" high seems about right, for a badly out of adjustment scope. The .30-06 is "flat" enough (as others have noted) that the error is going to be roughly proportional to distance...at least out to 100 yds.

Please note the discussion of "point blank range", which is the adjustment of a rifle's sights so that, up to that range, the point of impact (POI) doesn't vary from point of aim (POA) by more than the effective diameter of the target.

The way I understand this is, if you're hunting varmints with a 6" body size, you use that in point blank range calculations. If you are hunting deer with a 12" vital area size, use that in the calculations.

There are many ballistics calculators on line, free. Hornady has a good one, and there are others. If you look at typical ballistics for .30-06 w/ 150gr bullets, I believe you will have POI 1.5" higher than POA at 100 yds for a rifle adjusted for a "200 yd zero" (POI=POA at 200 yds).

I confess I do not understand your reasoning for this statement -

So I zeroed it at 100 and knew that at 50 it would be about 3 low.

With a scope mounted 1.5" above the bore and typical 150gr bullets at 2700 fps, the trajectory of a .30-06 is only 0.10" low at 50 yds, for a 100 yd zero. The bullet was 1.5" low when it started at the muzzle. How do you get 3" low for a 100 yd zero?

35 Whelen
October 16, 2008, 04:35 AM
I ran these numbers from this Ballistic Calculator (http://www.handloads.com/calc/) which should approximate a 30-06 with a 150 or 165 gr bullet:

Yards Impact
0_____-0.5"
25_____0.04"
50_____0.31"
75_____0.3"
100_____0"
125___ -0.61"
150___-1.55"
175___-2.82"
200___-4.44"

Your friend is full of the brown, smelly stuff.

35W

M_Olson
October 16, 2008, 10:43 AM
I confess I do not understand your reasoning for this statement -

Quote:
So I zeroed it at 100 and knew that at 50 it would be about 3 low.
With a scope mounted 1.5" above the bore and typical 150gr bullets at 2700 fps, the trajectory of a .30-06 is only 0.10" low at 50 yds, for a 100 yd zero. The bullet was 1.5" low when it started at the muzzle. How do you get 3" low for a 100 yd zero?

Thats a good point. Basically my reasoning came from what I saw (which may have been skewed as I'll explain in a moment). At 50 yards I was hitting ~2-3" high when aiming at the bullseye, then when I moved to the 100 yard I was hitting ~5-6" high. After zeroing at 100, I figured I would still be hitting ~3" lower at 50.

Also, due to a combination of my original post already being long winded and not really remembering until now, I left out the fact that for the first couple groupings at 100 I had the barrel resting on the sandbags instead of the stock without noticing. I would guess that would make me hit higher, and I did not go back to the 50 yd to compare, as I was out of ammo. There's also the fact that I'm not a very steady shot, even on sandbags, so those numbers may reflect me a lot more than they do the round.

Thank you everybody for your responses, they have been very helpful.

M_Olson
October 16, 2008, 10:56 AM
I looked up the information on the round I was using, which was the Winchester Super-X Silvertip, and I came up with 2700 fps, ballistic coefficient of .383, 180 grain bullet, and the scope actually sits about 2.25" measuring from the center of the scope to the center of the barrel (its one of those that sit high so you can use the iron sights underneath it). I plugged them into the calculator 35Whelen posted and came up with these numbers:

Yards____Impact
0________-2.25"
25_______-1.2"
50_______-0.47"
75_______-0.06"
100______ 0"
125______-0.3"
150______-0.97"
175______-2.04"
200______-3.51"

So it looks like I should be about half an inch low at 50 if zeroed at 100.

I like it so much better when I get information from people who know what they're talking about :)

Tony Williams
October 16, 2008, 11:12 AM
Given those figures, it looks as if you would do better to zero the rifle at a longer range than 100 yards.

If you zeroed it at 150 yards you would only be about half an inch high at 100 and 2 inches low at 200.

Zeroing it for 200 yards would give you a maximum height above the sight line of less than 2 inches (at 120 yards) with a drop of about 3 inches or so at 250 yards.

These figures are approximate, using the JBM program I mentioned earlier.

MTMilitiaman
October 16, 2008, 11:28 AM
The best analogy I have heard is that of throwing a football.

At close distance, a person doesn't really have to adjust much and can just launch the football to the intended target. But as the target gets farther away, it becomes necessary to launch the football up so that it falls on the target.

When you "sight in" a rifle for a particular range, you are adjusting the sights so that when you aim dead-on at the particular range, the bullet hits without compensation. This requires small adjustments to the angle of the bore in relation to the target.

If a rifle's barrel/bore is level, the bullet will lose velocity and drop due to gravity from the moment it leaves the muzzle until it strikes the ground. A bullet held level with the bore and dropped at the same moment the fired projectile leaves the barrel will hit the ground and exactly the same time the fired projectile hits the ground, even if the fired projectile obviously covers more horizontal distance, since both cover the same vertical distance.

Therefore, the bore must be angled upwards slightly so that the bullet it propelled over the line of sight and falls to its target. This is what sighting a rifle in entails.

So if a rifle is sighted in for 100 yards, it will be above the line of sight until it reaches 100 yards. It will reach the highest point in its arc at or slightly beyond "midrange," which is half the sight-in distance. So at 50 yards or slightly beyond, the bullet will be at its highest point above the line of sight and will then begin dropping to the sight in distance. After the sight in distance, the bullet will continue to drop at an exponential rate.

This means that the bullet crosses the line of sight twice--once before the sight in distance, and once at the sight in distance. The point before the sight in distance will rely on the flatness of the round's trajectory and the height of the optic above the bore, and will be as the bullet crosses the line of sight going up. It will continue to rise until it reaches the midrange, then will fall back across the line of sight at the zero range.

For most centerfire rifles producing velocities between 2700 and 3000 fps, with optics (from center of objective) 1.5 inches above the center of the bore, this means the bullet will cross the line of sight first between 20 and 30 yards, be 2 to 3 inches high at 100 yards, and dead on around 200 yards. So for years, our family has skipped bore sighting in favor of sighting the rifle to hit dead-on at 25 yards. We then move the target back to 100 yards and adjust until the point of impact is 2.5 to 3.5 inches high, depending on the exact round and load we are firing and the zero we want to achieve. This works for everything from the .270 and 7mm Rem Mag to the .30-06 and .338 RUM. I have my 7mm Rem Mag sighted in with 160 gr Noslers at 2960 fps so that it hits 2.5 inches high at 100 yards and dead on at 250 yards. This means my midrange trajectory never gets much above 3 inches above my line of sight, so I don't have to worry about aiming low on a deer sized animal at 130 yards or so.

The higher the optic sits above the bore, the longer it takes for the rifle to initially cross the line of sight.

Another example, in the Marine Corp, we established a Battle Zero (BZO) of 300 meters with the 4x RCO on the M16A4. To do this, we first zeroed the tip of the 100 meter chevron on the optic at 36 yards--this due to the fact that the optic sat fairly tall above the rifle's bore. Once this was established, shooters confirmed the 300 meter BZO by using the 300 meter aiming point at the tip of the stadia on torso pop up targets at 300 meters.

Hopefully, you aren't even more thoroughly confused right now.

I may not be an expert, but 14 to 17 inches high at 100 yards seems more than a little excessive for a cartridge in the power class of the 7.62x54 Russian to obtain a 200 yard zero. I would believe 1/3 or even 1/2 of that, but something is wrong and some adjustment is in order if you have to aim a foot and a half low to hit at 100 yards.

A friend of mine has a Tok semi-automatic rifle that fires the 7.62x54 round. With the rounds he was using, some silver-tipped surplus CZ rounds IIRC (about 150 gr), and a 200 yard zero, you have to aim at about the bottom of a Gatorade bottle to hit it at 100 yards. This is with a fixed 4x32 Bushnell wide view mounted as close as possible to the bore.

M_Olson
October 16, 2008, 12:07 PM
That was a very good explanation MTMilitiaman; thank you. Now, to apply it to the numbers I posted, are those numbers reflecting the trajectory of the bullet hitting 100 at the midrange point? Basically angled just enough to touch the POA right at 100 then begin falling back down? Or does it hit the point blank range (being the first crossing point of POI/POA, if I understand correctly) at 100 and hit midrange and the second intersection of POI/POA between 100 and 125 yds?

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