I got a MandA (or AandM) FSB from Brownells to fit the bull barrel on an AR15.  Fit just fine over barrel, gas tube went in just like it should, set screws screwed right in and set there like they should.  But the hole for the roll-pin to hold the gas tube was some weird size.  I ended up drilling it out to 5/64 so the pin would fit and engage the gas tube.

Waiting for reply from Brownells about such non-spec hole and wondered if anyone else had experienced similar.  

Actually, Brownells' first response was that their were no roll-pins as the set-screws held it on the barrel.  I had to "set" them straight that it had both screws AND a roll pin.