I used to have a DPMS heavy barrel upper and it was truly a tackdriver.
I now use and shoot a Ceiner with my three bushmaster 1/9 uppers. I have a 20" plain barrel ,16" with ak and a 16" superlight that was a plain barrel and has recently had a comp installed.
The 20" gets slightly better groups but all are similar.
My Ceiner is very ammo sensitive.CCI Mini Mags and most other high velocity stuff shoots about 2"+ at 50 yds (10 shot groups with a good scope on sandbags).I am useing a RR NM trigger.Factory triggers will limit you off the bench. Many of the real light aftermarket triggers will not run the rimfire guns well.Don't even think about a JEWELL, very nice and light in centerfire but will not run my Ceiner.
Ceiner directions caution to only use high velocity ammo but mine runs very good with target stuff. The only High velocity that groups well is PMC Zapper , I get about 1 1/4 " for the ten shot group @50 YD
CCI STD about the same as Zapper ,Fed 711 (target) 1 1/8" and WOLF MT shoots an honest 7/8" when I bear down and do my part. (and the wind isn't puffing)
My old DPMS would shoot much tighter groups, I think if I had a real good match trigger it would have shot match ammo into groups less than 1/2" at 50 YD but in practice I could hardly shoot more than ten shots into 7/8 because the 3 1/2 lb trigger would break funny and I was pulling shots wide.
Some ammo (dyna points and Rem GOLDEN bulk come to mind) shoot real wide-like 3 1/2" at 50 yd.
Bottom line is to try lots of different ammo including target and match to see what works best in your gun.This is a truth for all rimfire guns.
Keep in mind that newer Ceiners will more than likely need a bunch of high velocity stuff thru them to slick them up before they will run on standard vel ammo.Mine will even run faitly well with CCI sub sonic