Bullet Trajectory Question

[Hazcat]

I was just looking at the Federal Ammo 2007 Ballistics catalog and something jumped out at me that makes no sense.

The 30 Carbine trajectory is odd.  It says that if you zero at 100 it will also be zero at 50.  I think "Great, I'll zero at 50 and be good for 100".  Right?

Well then I reran it with a zero at 50 and it says it is 1.3 inches LOW at 100??

HUH?? I don't understand.  Zero at 100 = Zero at 50 BUT Zero at 50 is 1.3 low at 100.

Help!

[texcaliber]

it has to do with gravity.    I know you know that but if you zero at 50 vs.100 that means the bullet has twice the distance to be effected by gravity. 

here is a better way of saying it: zero@ 100yds. means 50yd zero is being hit on the incline of the trajectory and gravity pulls it back  down for the 100yd. zero.........where the 50yd zero gravity works from 50-100yds without the bullets incline producing the 1.3" low groups...... hope this helps Haz. Its a lot easier to explain on paper.   

Tex

[Hazcat]

Tex,

I know wha you mean but it still doesn't make sense.

If I zero at 100 I understand that the bullet is "rising" into the bull at 50 and "falling" into the bull at 100 due to the inherent trajectory (arc) of the 30 carbine round.

So when I zero at 50 the bullet should be rising into the bull and then fall into he bull again a 100. Right?

[texcaliber]

nope.

But i told you that it was easier to explain on paper.

Lets try it with sights and bore. If you sight in @100 then you are going to raise your front or lower your rear sight. And for 50yd zero you are going to zero it lowering your front raise/rear sights. That is if your range is level. Now repeat action but Your not looking at your sights any more. INSTEAD look at the centerline bore axis. NOTE the change of the bore centerline a and that should help explain the problem..............maybe not. But I am try for ya at least. 

[Hazcat]

Tex.

OK!  That makes sense.  I was not taking "bore sight" into consideration.

Thanks.

[Sponsor]

[Tim Burke]

Since the sights are above the gun, there is always an incline if the gun is zeroed at any range. When you launch a bullet upward at any angle less than 90°, the bullet will follow a parabolic arc. When you adjust the sights to incline the boreline just enough for the very top of the parabola to touch the Line Of Sight, you may be said to be sighted in for that distance. For every other distance, you will shoot below your point of aim. Now, adjust the sights to incline the bore line just a little steeper, and the top of the parabola will extend above the LOS. You are now sighted in at the point where the parabola crosses the LOS the first time on the way up, which will be closer than the peak of the arc, and again where the parabola crosses the LOS second time on the way down, which will be further away than the peak of the arc. Between those two points, the bullet strike will be above the LOS. Closer than the first zero, and further away than the second zero, the bullet strike will be below the LOS. If you adjust the sights to incline the boreline a little steeper still, then the first zero will be closer, the second zero will be further away, and the peak of the parabolic arc will be higher above the LOS.
Given the same ammo and the same height above the boreline for the sights, it makes no sense for a gun sighted in at 100 yards to be dead on at 50 yards, but for a gun sighted in at 50 yards to be 1.3" low at 100 yards. However, much of the discrepancy could be measurement error.

[Hazcat]

Given the same ammo and the same height above the boreline for the sights, it makes no sense for a gun sighted in at 100 yards to be dead on at 50 yards, but for a gun sighted in at 50 yards to be 1.3" low at 100 yards. However, much of the discrepancy could be measurement error.

I agree Tim,

But I ran it on The Federal Ballistics Catalog,  It's downloadable here http://www.federalpremium.com/ballistics/default.aspx (click the Fed logo on the right).

I also ran it on Remington Shoot! down loadable here http://www.remington.com/products/ammunition/ballistics/remington_shoot_ballistics_software.asp

On Fed it shows what I listed above, BUT on Remington it shows if I zero at 100 it is .920 high at 50 (the short zero is about 17 yards) and if I zero at 50 it is 1.998 low at 100

This whole thing is screwy.

I must say that in the Rem program you can set the height of your sights (.75 for the 30 carbine by my measurement) but you cannot in the Fed program.

[Tim Burke]

I'm on a Mac, I doubt if either program will work on my system.
The Remington data sounds to me like it might fit. Take a 50 yard zero up almost 2 minutes of angle to get a 100 yard zero, and it should be about an inch high at 50 yards.
The Federal data just confuses me.
I wonder what the Federal program assumes for the height of the sights above the bore, as that has significant effect on the relationship of the LOS and the bullet arc.

[Hazcat]

I'm on a Mac, I doubt if either program will work on my system.
The Remington data sounds to me like it might fit. Take a 50 yard zero up almost 2 minutes of angle to get a 100 yard zero, and it should be about an inch high at 50 yards.
The Federal data just confuses me.
I wonder what the Federal program assumes for the height of the sights above the bore, as that has significant effect on the relationship of the LOS and the bullet arc.

Tim,

If I reset the sights for 1.5 (seems to be the standard) on the Remington then it is .713 high at 50 with 100 zero and 1.478 low at 100 with a 50 zero.

[Tim Burke]

Those seem like reasonable numbers, too. The bullet should be roughly twice as low at 100 yards with a 50 yard zero as it is high at 50 yards with a 100 yard zero.