Last weekend I built a new lower, and since I had an A2 stock screw in it where I needed an A1, I thought I'd function-test it with an M261 conversion (no issue with the buffer hitting the screw). Bolted up my old parts-gun A1 upper and shot 8 groups with each of two kinds of .22 ammo. If anything, the old 1:12 upper/M261 combo seems to like this lower better than its old one. Groups ran like this (5 shot groups @ 25 yards):
Federal Walmart Bulk:
1.080
.604
.645
.594
.324
.726
.332
.510
Average of the six best groups: .394"
Winchester Dynapoints:
.371
.748
.797
.382
.616
.262
.283
.198!!!!
Average of the six best groups: .352"
And many of the groups were 4+1s, too, with one flyer spoiling an otherwise very tight group.
If I hadn't shot these groups myself, I wouldn't have believed a conversion capable of this level of accuracy. Usually the M261 will shoot about 3/4" with the Federal and about 5/8" with the Dynapoints with a few groups just under 1/2". This is by far the best accuracy I've ever gotten with my conversion unit--neither of my 10/22s and at least one of my bolt actions don't shoot this well! It shoots ALMOST as well as the DPMS .22 M4--but in honesty I should note that the DPMS shoots a 3/8" average much more consistently. Look at the numbers above and you'll notice there's a fairly large spread.
Still, I'm very happy with how this project came out, especially since my kid has pretty much hijacked the DPMS .22 gun. Since I wasn't using this upper for anything else, I think it just got "dedicated." Now I have one to shoot too!