I recently bought a used M&P9, and decided to run a few snap caps through it to get familiar with working the action, etc. To my surprise, the slide would not close all the way. I then tried putting the "proof" brass in, and got the same result. After reading the instructions, I took the slide off, and removed the barrel. I measured from the back face of the barrel to the "step" where the brass would stop, and it's only 15.5mm deep. The top of the barrel is stamped 9mm, but no other markings. I'd like to get some more information before I bring it back to the dealer. Thanks in advance.