The MFR has the bracket on the inside of the rear of the rail, which gives it much less clearance to fit over the gas block during installation.
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I guess some clarification. The MFR mounting ring will not fit over the gas block. You must slip the ring onto the barrel nut, install the gas block, mount the float tube to the mounting ring, then fasten the mounting ring to the barrel nut. Using this method the rail clears the JP block fine.
I have assembled 4 uppers 2 of which are SBR's based on BCM 11.5" lightweight barrels. Total round count is in excess of 10,000 rounds and there have been zero issues with these item(s.)
I left about .035" clearance from the barrel shoulder at inception, and the feeler gauge still reads .035" for the space on my SBR. This is a very strong mounting solution.
http://i181.photobucket.com/albums/x...115inchSBR.jpg