Hey Dean, I just owned (and sold) my CMMG MK3 with an SST 18" upper. Probably the best configuration for multi-purpose rifle ever, in my opinion. But notice, I SOLD it?
They made a manufacturing error or bought barrels with an incorrect gas port which 'overgasses' the system and results in perpetual FTEs. (Eject)
This statement is not made to skew them, as I like CMMG very much. The gun was super accurate and had a lovely fit and finish. Their attempt to ameliorate the problem was very odd; instead of the correct gas system, they were treating 'symptoms' of overgassing. (Rifle length stock, spacer in the buffer)
But, until they come out, fess up and fix their mistake, turn and run the other way. You will save yourself a ton of headaches and UPS RAs to Missouri.
IF you'd like more info, I can forward you a link to another forum where it is discussed in detail.
Rock River is nice, super accurate, but I do not like FAL mags. Anything with an SR-25 pattern mag; I.E. Magpul Pmag! DPMS...