The shank of the screw needs to be large enough that one half of it is engaging the collar, and one half engaging the bushing, with enough engagement to hold the barrel in. I suspect that a number 10 screw would work. Original would have been BA; perhaps a 10-24 05 10-32 would suffice. Or a 8- could be tried.
The reason I ask about registration status is that this thread is about making a legal reroduction of the Sten. Not entirely convinced that posting a photo of a currently unregistered gun is all that helpful.
