Hello,
From what you mention I assume that you only want a gas-piston (GP) upper. LMT is great, but fairly new. Barrett, of course is great, but very expensive and also quite new. Lastly, LWRC has been around the longest (with GP designed firearms), plus expensive.
So here are my thoughts; LMT and Barrett are great firearms but let them manufacture these firearms for a few generations to correct any issues with the piston design. I would personally get an LWRC. LWRC only makes GP firearms and has been doing for a long time. The only problems that LWRC has is MAKING ENOUGH FIREARMS. Seriously, I own several LWRC M6's both in 5.56 and 6.8 and they are solid, tought and just want die. So IMHO get an LWRC now, or wait for LMT and Barrett to run their designs long enough to correct any issues.
Before anyone crucifies me, Barrett has made the Rec7 for a couple generations but it still has problems. Barrett is new to 6.8's but not to 50's.
Hope this helps (Also Hi, I am new to the forum :mrgreen: )
Jeremy