Re: Which piston, LWRCI, POF, Sig556, FN 16S, ACR, LMT
Sig556 has everything going for it except accuracy, due to the long piston design and thin barrel, at best 2.5"-3" groups. This is match 69gr, 75grn and 77grn ammo, I use since it is 1:7 twist. And before someone would say the accuracy is partial to my particular sig, this was conclusive after testing three different Sig556. I had one with a Samson Rail handguard, fishgill, and Sig551 handguard. What we find is the accuracy of the rifle gets better if nothing touch the rail (including your hand). Using 55gr and 62 gr yeilds 4"-6" grouping.
The LWRC M6A2 and M6A3 proved consistent with all weight bullets 55gr-62gr avg 2", inproving to 1.25"-1.5" with the 69gr and heavier bullets. This is due to the heavy barrel.
FS2000 is in between the Sig556 and LWRC rifles, avg 2"-3" groups, though I don't see this as a long range rifle, more of a 100-200 yard gun.
I only can comment on these two as they are the only piston ARs I own. Hope it helps your decision, if it was up to me, I would stick with the LWRC or FS2000