Hi oldguy! Thanks for the gas leak test info... sounds like a simple and effective test! The carbine barrel that I am refering to was introduced, I believe, in Viet Nam. I would assume for helicopter crews and such.. Bushmaster reintroduced this same barrel in their original dissapator barrel (sp?) prior to going to the same type port as a regular 16" barrel... I see the DPMS catalogue had the same offering in an FN barrel, but it has been discontinued.... I had a early Bushmaster barrel of this type, but felt it was too heavy for my liking, so I have been looking for a military version of this barrel for some time, finally found one... but not sure if it is up to spec... or for that matter, if it is even original.. but, they do exist! I am going to clean and polish the chamber this week end... I think that it could be the problem, as has been suggested here on this forum... Thanks for the neat test suggestion, and best of luck to ya! til....later...GT