looks good, but um 12.7mm is a little...well, it's a lot big of a round for this lol, 12.7 is huge. a pistol this size takes maybe a 2.5 to a 4.5mm and that would be pushing it. just little details that you may want to make more realistic :) and it would never be automatic. 12.7 is for a .50cal machine gun.
I found your functional magnum pistol on Thingiverse and uploaded it to my Tinkercad account. Which License would you like me to put it under before I make it public? Since it is your design, I didn't want to treat it like it was mine and not consult you before I make a decision. If you need to look me up, my avatar name is Kardanukan.