As discussed here, I decided about a year and a half ago to build an AR for shooting off the bench. No specific reason why, I just wanted a range plinker.
I've had a few missteps and changes along the way, but this is what I ended up with:
Anderson Lower
BCM Blem Upper
Larue 20" Stealth Barrel, 1:8 twist, Wylde chamber, no muzzle device
Brownells BCG
BCM Charging Handle
Larue Trigger
Generic lower parts kit
A2 Stock, buffer tube, and standard weight buffer
Adjustable gas block with gas turned down so the gun just cycles and the bolt locks back when the mag is empty.
Aero Precision Quantum Handguard
Magpul Bipod
Burris 3-9x40 scope
Nikon scope mount
Magpul pistol grip
Magpul 10rnd magazine
Missteps were mainly using a different upper receiver that was on the large end of spec while the Larue barrel's extension was on the lower end of spec, resulting in a very sloppy barrel/receiver fit. Even using green Loctite and greater barrel nut torque (per Rob at Virginia Arms) didn't get me where I wanted to be (best it could do was 1.5"@100yds). I bought a BCM blem upper and a better handguard/barrel-nut and that's where I am today.
What I have now is a gun that will shoot most commercial loads into 1.5" or less for 5 shots. It'll shoot most 73gr+ loads into less than an inch or better for 3 shots and just under to just over 1" for 5 shots (Nosler SSA 75gr is sub 1" even for 5 shots). I'm surprised at its preference for heavier bullets as I expected the 1:8 twist to be more ideally suited to bullets in the 60-70 range. Even stranger is that I found a couple 55gr loads it likes about as much as the heavy bullet loads. It doesn't like anything I've tried in the 63-68gr range. If the bullet weight starts with a "6", it's wrong apparently.
This is what I shot this morning (the loads it seems to like):
American Eagle MSR 55gr: .89" average for 2 3-shot groups, over 1.5" for 5-shots on my last trip
Nosler SSA 75gr OTM: .66" average for 2 3-shot groups (also did sub 1" 5-shot groups my last trip, but I don't recall the exact measurement)
Hornady Match 73gr ELD: .931" for 2 3-shot groups and 1" for 1 5-shot group
PPU M193 55gr: .716" for 2 3-shot groups, but was well over an inch for my last trip where I was shooting 5-shot groups
Hornady Black 75gr BTHP: .553" for 2 3-shot group and 1.5" for 1 5-shot group. Could be shooter fatigue as that was the last of 17 groups.
It does not like loads with bullets below 55gr or in the 60-69gr range. It doesn't like most 55gr loads, but did well with the AE MSR load for 3-shot group (did poorly with 5-shot groups on my last trip).
Question time:
1. Should I do anything with the gun? While the barrel fit this upper better than the old upper, it's still not snug (ie I didn't have to heat the upper for it to slide in). Should I Loctite it?
2. The Aero Precision instructions provided a torque figure for their barrel nut lower than most folks suggest, presumably because it's aluminum instead of steel. Should I torque it to a higher value anyway?
3. Does an AR need to "break in"? It seems to be getting more accurate, but maybe I'm getting better at shooting it.
4. Is there a specific technique to shooting ARs off the bench? I did notice if I put too much downward pressure against the bipod, groups would shift. Once I started minimizing this pressure, my groups would be more consistent and smaller. I was using the bipod up front and a sandbag in the rear.
5. What is the "correct" number of shots in a string to judge accuracy? Three is typical for a hunting rifle, but what about an AR that is used for target shooting? Five? Ten?
I'll be happy if the consensus is that I just need to learn how to shoot the gun. I don't have a lot of experience with centerfire semi-autos, so if the technique is different, I'm ignorant.
Chris