A common procedure for slugging a barrel is to press a lead slug through it. For a 22 cal you can break apart a few 22 rimfire cartridges, or you can buy them...search the Rimfire Forum.
A CAUTION--for breaking apart 22 cal rimfire, remember that you are dealing with a live primer.
For 6mm I took an old 22 cal bullet mold and reamed out the cavities to 0.245". I'm an old cast bullet shooter so I have a bunch of molds, melt pots and bullet material including pure chemical grade lead (Chemical lead is 99.95% pure).
Your barrel needs to be clean, no powder residue or copper. Oil the bore lightly before slugging.
Slugging a chambered 6mm barrel, I lightly grease the slug, then insert it in a soda straw, available at Hardys, just the right side. This assembly fits right up in the neck of a 6mm chamber. Then, using a Dewey cleaning rod with the bare end, I gently slide the bullet down the barrel.
According to what you are trying to determine, you may have to slug in both directions. i.e. If you find a tight place, say, 6" from the muzzle, push the slug all the way through from the chamber. Then measure the slug. Then slug from the muzzle to near the tight spot and measure that slug.
Pretty soon you will learn to feel what the barrel is trying to tell you. Yes, the barrel will actually talk to you if you know how to listen.
