I have 16" dissapator upper that I bought off someguy a long time ago.  It  would make me mad once in awhile as the the "gas block"? would shift down while shooting and not cycle. The allen keys tightened all the way with blue locktite wouldnt hold it.  Its a diffrent gas block than what is pictured on current bush master catalog/website. I didnt sweat over it too much as I wanted a M4 setup anyways. Does anyone else have one of these that is giving them problems?  How do you fix it?

I'll give you $20.00 for it, if it's that big of a problem.

is the gas block pinned onto the barrel?  If yes I can't imagine how it can shift around.  if not, perhaps pinning the gas block will fix the problem.

p.s. did you receive my messages in response to your email regarding the FMP build?