My (non technical) thought was to have the body of each gun in two parts with a hollow (when assembled forming a socket) that takes a length of the turned/ rod barrel to provide strength to the exposed part of the barrel. Scale plastic barrels would just need a stub piece for assembly.
It might be possible to make the plastic barrels a friction fit, making replacement easier.
Ripley makes a good point about the flash guards, an alternative would be to provide the socket (so no need to drill the body of the gun) and flashguards in plastic as spares on the sprue.
No idea of the feasibility, but the Crusader kit had a wire brush guard option, a first for this scale.
Technical issues are preventing me from posting a diagram.