I could use some help. Are any of you ballistics savvy?
I've been having an accuracy/consistency problem with my AR. This is the second scope I've had mounted on this rifle, so that should be ruled out. Also, the mount has been torqued and re-torqued for good measure. This rifle has a 24" bull (target grade) barrel with a 1:8 rifling twist and a 5.56x45mm NATO chamber.
- So far, I've only shot .223 REM 55 gr. FMJ out of it. I know that can produce some accuracy shortcomings with a 5.56 chamber.
- I've been shooting 3-5 round groups at 100 yards. I can generally put 2 rounds in nearly the same hole (expected) but then the 3rd ends up a few inches from the first too. Something about that isn't right. When I should be grouping all rounds under a quarter and that distance, the groups are more like baseball sized. Not good.
- I am hand feeding every round into the chamber with no magazine inserted, removing the possibility that magazine spring pressure is pushing up on the underside of the bolt and causing accuracy issues.
- The chamber and barrel are clean, so there should be no bolt face seating issues. However, to eliminate that possibility, I am slamming the bolt closed and hitting the forward assist plunger 3 times before each shot, to ensure consistent seating.
- A 1:8 twist *should* be just fine for 55 gr. bullets, but that assumption is usually made with 16-20" barrels. Should a 24" barrel with the same twist rate change that much?
- I ordered a bunch of different 5.56x45mm NATO cartridges with different bullet weights to try at the range. I'm going to be using a Caldwell Lead Sled shooting rest from a bench (as pictured above) at 100 yards. This is a very stable shooting platform, from which I can literally fire the gun with one finger, allowing for very precise experimentation. I will also choose a wind-less day, though wind should not be much of a factor at 100 yards.
- Bullet selection is as follows:
Federal Ammunition 5.56x45mm NATO, 55 gr. XM193 Ball FMJ
Federal Ammunition 5.56x45mm NATO, 62 gr. XM855 Ball FMJ w/ steel penetrator
Barnes VOR-TX Ammunition 5.56x45mm NATO, 70 gr. Triple-Shock X lead free Hollow Point
IMI Ammunition 5.56x45mm NATO, 77 gr. Sierra Match King, Open Tip Match
Barnes Precision Match Ammunition 5.56x45mm NATO, 85 gr. Open Tip Match
- So that gives me 55, 62, 70, 77 and 85 grain bullets to try out. 85 is probably a little excessive for this rifling twist rate, but I was tempted to give it a try.
What bullet(s) do you expect to group best and why?
Is there anything else I should be doing that you can think of, to not only produce tighter groups but also perform the most scientific experiment possible?
