MMcfpd
Member
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?
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?