The best barrel extension is going to be some 4130 dom tubing from a special metal supplier like wicks or spruce aircraft ect. You can pick a size thats the same or close to the outside of the muzzle of your barrel and an inside diameter that is over your bore diameter. Drill out the last inch of the tubing and turn the last inch of the barrel down for a light press fit, then weld it on. Here's a suomi barrel I lenghtened.
How much does that usally cost Moleman. Looks like a cool Idea
1' of the tubing is about $6 depending on what size you go with. I went with 3/4"x .156" wall which gave me a .437" id for my suomi builds. That way the bullet has plenty of wiggle room to get out of the barrel without touching the extension.
there is a guy in n.c. that makes new barrels i think.