RPF PREMIUM MEMBER
He probably can do that on his mill. He threaded the inside of the large nut that he made. rifling the barrel would probably be similar.
I'm no gun expert but from what I've seen online, rifling isn't something you can just put on a mill to do. You could probably do a faux-rifling that way but at this point in his build, I think he'd do it for real and rifling bits are these small things you have to push through the barrel for it to etch the lines.
I'm really interested to see what he comes up with.