During WW2 motion picture companies turned over rifles and machineguns to the army and national guard, mainly for use in training.
Most bolt action rifles that had been used in WW1 movies had not been altered and could still fire live cartridges, though barrels were often corroded past safe firing.
Machineguns had to be altered in order to fire full auto with blank ammunition, but could be put back in service by replacing the barrel, which for Browning and Vickers/Maxims was the easy part.
In an article I read long ago a veteran wrote of the setting up of a roadblock at a strategic industry.
They received an MG still in the crate, and on opening it they found a blank firing movie prop complete with belted blanks.