Trying to figure out "what is it?" is fun, but don't confuse Hollywood and TV guns with reality. A bunch of guys make their living building faked up guns for the big and small screen. Sometimes the guns will fire blanks, others are pure phonies with the noise and sound provided by the backroom boys. Sometimes, they start with a real gun (as was done with this one), other times, they just make a wood or plastic prop.
Nor are guns always appropriate to the movie. We all remember the Star Wars troopers carrying MG.42's and Patchett SMG's painted white. And a faked up C96 Mauser that Han Solo carried.
Jim