You mean like this UZi-MAC hybrid? LOL! From Dogs of War (1980). It looks like they did have several real uzis, but for whatever reason used one of the obvious fakes for a split second close up.Hope this isn't taking the magic out of anyone's movie experience but we used to build movie props and lots of weapons in battle scenes weren't much more than pipe (sometimes conduit) and a rough-shaped 2X4.
Lots of stuff in crowd scenes are panned past so quick that details disappear in a blur. For close-ups, the three or four "realistic" or blank-firing pieces are just passed around throughout the various scenes. Now, with CGI, it's all just copy and paste against a green screen.
We hung a door on a shed that turned out to be about 4" too narrow for the door frame; but since the door was always shown standing open, the director said 'who cares'... and he was right. Couldn't see it even when we knew to look for it.