As far as anyone knows, the firing props in the UK were rented from Bapty's and the stunt blasters were casts - as stunt props usually are. Then one was cobbled together for a close-up, which was shot in the US: the "Greedo Killer". If this blaster had existed in the US, then this would probably have been the Greedo Killer, but it isn't.
We know the Hero blaster was cast, and that casts (in various configurations) were worn by Imperial Officers and used by Han Solo at least in subsequent movies.
The "bull-barrel" is a unique mod to a specific Mauser C96 upper, and I doubt that Lucasfilm would have replicated that by modifying a real C96. they usually didn't go to that level of detail.
Some owners of authentic Star Wars movie-props that are missing parts have added similar parts (authentic or replica) to them to make them look like they once had been, but I don't think many collectors would modify a genuine movie prop in that way because that would lessen its value.