Starting w/an octagon bbl I'm presuming,,
Of course doing it on a lathe that's large enough to handle the bbl length is no problem. The set-up would probably take more time than the run.
But you can do it with nothing more than hand files.
The octagon flats give you a good start towards your goal.
Mark off your 3".
Then hand file the points or edges of the octagon bbl to flats till you have 16 equal width flats. Then take those down to 32 equal flats,,ect.
What you end up with is a round bbl.
Done carefully the end result can be very precise.
A simple slip gauge,, a piece of tubing (brass, steel, whatever) can be used to fit & file for the final precise shape by smoking the bbl and sliding the tube on and off the bbl to reveal any high spots.
Then use that slip gauge (or another tube with same ID as your socket for making the bayonet.
If your bbl is a swamped bbl and not a straight octagon,,file those 3inches at the muzzle to straight octagon first, then to progressive flats to attain the round shape.
Well, that's the way I'd do it anyway,,
With good sharp files, 12L14 steel,, it doesn't take as long as you'd think to do stuff like this.
