62 gr. v. 75 gr. in an AR-15?

[MMcfpd]

Last weekend I took a 20" AR-15 to the range and after firing a magazine of the 75 gr. .223 with which it was zeroed, I tried, for s&g, a magazine I had with me of IMI M193. The gun was fine with the 75 gr., but the M193 was way off, low and left. Enough to make me wonder about IMI ammo.

So I decided to try again today with the same gun, the same Hornady 75 gr. steel match and some Federal M855 (62 gr.). Firing half a magazine (10 rds) of the 75 gr., all were in the bullseye (50 yds), while a half magazine of M855 grouped OK, but ~3" low (and a tad left, but there was some wind).

Then I finished both half magazines with a 14.5" barreled AR-15, and both types of ammo grouped together at bullseye height. Both the barrels have a 1:7 rifling twist.

I was surprised to see such a significant difference with the 20" gun at just 50 yards and almost no difference with the 14.5" gun. My guess as to what I witnessed is that the difference in muzzle velocities between the two weights of ammo is great enough to see the observed result from the 20" gun, but with the 14.5" gun the shorter barrel doesn't produce that big a difference in the bullets' muzzle velocities. Still, though, if anything beforehand I might have expected the heavier bullet to be the one to come in lower on the target.

Other explanations?

 

[Z-Michigan]

The SS109 bullet in M855 ammo is very long and almost as long as typical 75gr MHP type bullets (it's identical in length to a roughly 70-73gr MHP bullet). It also would be much closer in velocity and somewhat closer in BC to the 75gr. The 55gr FMJ bullet is much shorter than either one, has a high velocity even in short barrels (especially IMI which tends to be among the hottest) and, although it probably doesn't matter at 50 yards, a much lower BC than either one. I'm not sure that fully explains it, but long and short the M855 is a lot closer to the 75gr than the M193 is.

 

[Samclrk]

I`m shooting a 218Bee..With fifty grain VMax,it will hit dead center.With fifty-two grain Sierra`s,it will be two inches two the right.This is a hundred yards.Forty grain VMax will be center and high.

 

[helotaxi]

The harmonics of the barrel are going to determine where each different round hits. At 100yds, BC and velocity aren't going to have much of an impact (so to speak).

 

[briansmithwins]

And some rifles just don't like some types of ammo.

I had a AR with a 1:7 chromed barrel that would get 1MOA groups with PPU 75gr ammo. With 55gr XM193 I could get 1.5MOA.

All the 62gr ammo I tried (not cheap comie steel either) looked like I was shooting a shotgun.

You pretty much have to try your guns and see what they do. Generally 1:7 barrels wil be better with heavier ammo and 1:12 are better with light bullets.

BSW

 

[Pat M]

I fired two groups last weekend at 100 yds, one with Hornady 55 gr. FMJ (for which the rifle was zeroed), and the other with Sierra 69 gr. Match Kings. This was in a 20 inch 1:9 Armalite. The 69s were in a group the size of a nickel, but the point of impact was about 5 inches low. Keep experimenting to see what shoots well.

 

[35 Whelen]

the harmonics of the barrel are going to determine where each different round hits. At 100yds, bc and velocity aren't going to have much of an impact (so to speak).

+1

35w

 

[helotaxi]

I had a AR with a 1:7 chromed barrel that would get 1MOA groups with PPU 75gr ammo. With 55gr XM193 I could get 1.5MOA.

All the 62gr ammo I tried (not cheap comie steel either) looked like I was shooting a shotgun.

Sounds a lot more like a case of good ammo vs. crap ammo but whatever. Mil-spec bullets are not selected nor noted for their precision. The Privi 75gn uses match bullets.

 

[rcmodel]

Yep.

If you can get 1.5 MOA with any military FMJ you are doing way better then the acceptance spec for the bullets.

Bullets with the jacket hole in the base are never as accurate as bullets with the jacket hole in the nose.

rc

 

[briansmithwins]

Sounds a lot more like a case of good ammo vs. crap ammo but whatever. Mil-spec bullets are not selected nor noted for their precision. The Privi 75gn uses match bullets.

Actually, the 62gr stuff was PPU also. Not what I had expected but the targets ont lie.

BSW

 

[chaser_2332]

Was the groups bad or just poa/poi off? You were only at 50yds I can't imagine anything doing bad at that distance.

 

[helotaxi]

Actually, the 62gr stuff was PPU also.

Still uses a bullet neither selected nor known for its precision. It might be a terrifically consistent load with tiny standard devs and extreme spreads, but with an open based mil type bullet it is still not a match quality round and is still perfectly suited to combat use. 5 MOA is the required 100yd precision for the M16/M4 weapon-ammo combo, IIRC. Most of the rifles are capable of way better than that but feed the best rifle crap ammo and as you said, the targets don't lie.