I have 2 of them, Sig 556 swat and one with the 2nd gen polymer rail. Just sold the 7.62 version 556R (Russian).
They THUMP, so solid, piston driven. Highly suggested.
Got a box this morning!! Will drop at Outerlimits in Holbrook, M&M in Plymouth today, Gartman tomorrow.
I'll save some to get to the Holbrook gun show too