I wanted to know if anyone has changed barrels on a LMT CQB-MRP Piston upper from a 5.56mm to a 6.8mm.
I saw that LMT is now offering 6.8mm barrels, however I wasn’t sure if you could use the 5.56mm piston bolt carrier group with a 6.8mm bolt assembly.
The bolt need to be changed. Diff. size case rim.
So I can use a 6.8mm bolt, and use the same bolt carrier assembly (the piston BCA)?
I wasn’t sure if the 6.8mm barrel works with just direct impingement MRP, or if it worked with the piston one as well.
LMT sells a complete 6.8mm bolt carrier group for the direct impingement MRP, however they don’t have one for the piston MRP.
You can switch between 5.56 and 6.8 by changing the barrel and bolt for DI MRPs.
However, for piston guns (or MRP piston conversions) the piston system comes attached to the barrel and the piston bolt comes with the kit. As far as I know, LMT is not yet selling the 6.8 MPR Piston unless it was just announced. So far, you could only use 6.8 DI.
I would also rec exploring an H2 buffer (All recent LMT’s I’ve seen come with an H buffer to start) or maybe even an H3 if you’re going with the 6.8.
I don’t know anything about the piston 6.8 kits. I know the DI kits only require a change of bolt. I suggest an email to LMT.
BTW, 5.56 piston conversion kits which only take about 2 minutes to change out for the DI system, comes with an H2 buffer.
I did not know that. LMT is with it. NO shock there. 
They do it right.
BTW, I just checked out their website and it appears that they are selling 6.8 piston rifles! This is new. However, it doesn’t look like they have 6.8 conversion kits yet. But I suspect they will get those out pretty quick.
From LMT’s website:
CQB MRP™ Upper Receiver with a 16" Chrome Lined 1:10" Twist 6.8 Piston Barrel, Semi-Auto 6.8 Bolt Carrier Group, Tactical Charging Handle Assembly, Defender Lower with Sopmod Buttstock and Standard Trigger Group. Ships with a Sling, Operators Manual, Tactical adjustable Rear Sight, Tactical Front Sight, (1) Twenty Five Round 6.8 Magazine, Torque Wrench / Driver and (3) Rail Panels. you must have a current Federal Firearms License on file to purchase complete weapons or serialized parts. Piston Operated Overall length with SOPMOD collapsed: 33" Overall length with SOPMOD extended: 36" A2 birdcage compensator 1/2 X 28 thread pitch Low profile gas block Cryogenically treated barrel to increase accuracy, barrel life and easier cleaning Electronically tested and recorded trigger pull
I just got an e-mail back from LMT which said: “All you would need is the Barrel, Bolt, and Magazine.”
So apparently, I can use my 5.56 bolt carrier group with a 6.8mm bolt.
Being able to change barrels/calibers is a nice option to have on the LMT CQB-MRP uppers. I’m definitely thinking of converting mine to 6.8mm in the near future, I’m just waiting on a 14.5" 6.8mm barrel to be released.
are they considering making the 6.8 in 14.5?
I just received a 12.5" 6.8 SPC barrel.
Wait for the 300 AAC Blackout and you can use the same bolt and magazines!