I'd bet he's using a regular carbine receiver extension. In that case, there's no screw to obstruct the travel distance, only the back end of the tube. My money is still on a lower or parts kit issue though.Shit - could be a short tube, but cutting the screw down worked. I replaced the damn mag release with 2 others, swapped uppers, etc. Finally occurred to me that it had to be somewhere in the extension just by process of elimination. Sounds like this could very well be the OP's problem as well.