What are reasonable accuracy expectations from an 18" medium weight 1:7 barrel, with a variety of loads, ranging from inexpensive 55 gr FMJ to factory match grade 77 gr bullets?
For reference, I’m considering an LWRC M6A3 DMR (18 inch barrel, 1:7 twist). I think the supersonic range is about 500 yds for 55 gr and 800 yds for 77 gr bullets. How accurate will these be at these distances? Should I expect 8" groups at 800 yds using factory match 77 gr ammo (Black Hills SMK, for example)? How about 8" groups at 500 yds using inexpensive factory 55 gr ammo (PMC Bronze, for example)? If those are realistic expectations, I may follow through on this purchase.
Would there be much of an accuracy difference compared to the standard length 16 inch M6A3 (non-DMR)?
Generally speaking a very accurate AR will be able to shoot 3/4" groups reliably. You’d be extremely hard pressed to find a gun that will shoot 1/2" groups repeatedly.
You can’t really translate 1" groups at 100 yards equaling 8" groups at 800 yards. A lot more happens to the bullet between 1-800 yards than 1-800. Wind, bullet stability, and the fact its just plain hard to see the target at 800, and put the cross hairs on the same spot shot after shot.
Again, generally speaking, 55GR FMJ type rounds are 2-4" depending on the gun. Out of my LaRue Stealth I can get IMI M193 into 2" groups at 100 but it takes good shooting on my part, and a calm day. Out of rack grade guns that would be more like 3" groups at best. The stuff is simply designed to be mass produced, and achieve combat satisfactory accuracy.
In regards to match grade ammo you really need to experiment and see what the gun likes. SSA 77GR 5.56 ammo shoots well out of my Stealth. Today I shot some groups with it at 200, and was getting between 1.75" and 2.25" groups. Hornady 75GR match ammo was shooting around 2.5-3", and Prvi 75GR match was pretty bad at around 4-5". This was with blustery wind from 5-15MPH, and with only a 4x max power optic.
Barrel length doesn’t really have anything to do with accuracy, and some can argue a shorter barrel is better for accuracy because there is less flex in a shorter tube. What barrel length has most to do with is velocity, and you gain roughly 50FPS for each inch of barrel.
I would also stay away from a piston gun if you are looking for accuracy because they are known to decrease accuracy. They put moving parts on the barrel itself, and can mess with the harmonics. Sorta like the difference between a free float handguard, and a non free float handguard. You don’t want anything touching the barrel (or as little as all possible). I’d look at factory uppers from either Noveske or LaRue. You can also put together an upper either yourself or from a semi-custom shop using your own list of parts.
