Guns for movies are supplied by a few rental companies geared specifically to the industry.
I believe there is a big one in NYC near Soho as well as the most famous one out West which is Stembridge.
These companies are licensed and given exemptions by their respective States (and the ATFE as well?) in order to supply guns for the movies. Otherwise, it would be illegal for some of the actors with felony convictions to handle them.
AFAIK most, if not all, are original guns that have been adapted for blank fire. Not as many cheesy ones as in years past. The rental companies employ machinists to 'build' custom models as needed. Of course, they also supply non firing guns as props as well.
Tight security is maintained at these businesses.
Most of the guns would require rebarreling at the least to be operational with live ammo.
Thieves aren't in the habit of burglarizing guns that need extensive machine work to fire.
There is no pressure that can be brought to bear against companies that do not do any business at all with the public.
Any anti-gun legislation ALWAYS makes exceptions for the movies.
The film industry is far too powerful in income generated in this Country to ever hear the word 'NO'.
Best you could do would be to attend a MGM or Universal stockholders meeting with your concerns.
Charleton Heston did just that at a Time/Warner meeting 12 years ago. Gave him some satisfaction, but no change in company policy in the long run.