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!

I too have been very happy with my Ceiner in a bushy 1/9.
If you can get it to run smooth try some standard vel ammo.
Ceiner suggests only high vel copper plate but mine runs with any standard and will even run CCI SUBSONICS half way decent when clean.
CCI Standard and WOLF MT both have given great groops for me,I have been playing at 50 yd and even at 100 yd
PMC zapper has been the best high vel round for me.
Results vary wildly from ammo to ammo. How can .22 ammo like rem golden shoot 2 1/2"-3" ten shot groops @50 yd and wolf MT will go under 1" all day long ? Yeah the goldens cost about $17 per 500 and the Wolf MT are  $30 but still is a hugh difference. The PMC Zapper is only a couple buchs morethan the rem golden and shoots groops half the size for me