I just got an IBM Carbine that was imported by CAI. I'll have to vouch for what Frank said about them being dirty. They must have had a rule that said to oil all parts heavily everyday! I'm beginning my second day of the detail cleaning and have never seen as much "crud" inside of a gun before. There's a tab on the slide near the back that runs in the receiver rail. From top to bottom, this tab is supposed to measure around .240 in. on a good one. If it's worn a lot, that might be your problem. I had one that was worn down to .226 in. and it disengaged pretty easily. Other places to look for wear are on the receiver groove itself and on the grooves on the sides of the barrel, including the lugs on the slide that run in them. The day I bought my IBM, they had an Inland with a better looking stock, but it had a nasty looking dark ring inside the barrel about an inch or so back from the muzzle. My IBM has a nice NPM trigger housing. Does your NPM happen to have an IBM? Welcome to the Forum! - Bob