I think that most here will respond that it is an attempt to mount a moderator ['silencer']...
But many years ago, I read an article with a bizarre comment that mentioned a pistol with a threaded barrel that was used to set off a 'scuttling charge'...Apparently you would screw the barrel/pistol into a 55 gallon drum full of explosive and set it off with a blank or incendiary ['tracer'] cartridge...Since the trigger would need to be pulled, I can't imagine this setup being very popular...
I know, it sounds preposterous...Take it for what it is...