It's been many years ago, but I cut off a .22 barrel, then carefully filed and sanded it square, then lapped it with a round head brass screw in an eggbeater hand drill. It came out very nice. One tricky part is avoiding burrs at the edge of the bore. I checked with a clean q-tip that would catch fibers on any burr that was present. I've had crowns done by gunsmiths fail that test. Then there was the smith that crowned another .22 for me, and left so much burr that I couldn't get a cleaning rod through it, but that's another story.
I've done this myself too on several "beater" rifles (although I used a cotton ball instead of a Q-tip to test for burrs!).
I had the opportunity to do an 'after" test on 2 of the 4 centrefirel rifles that I did this on for friends, and one (a Rem 700) shot MOA, the other was a Savage 99 (before a very slight shortening, the muzzle was clearly worn egg-shaped by I suppose a cleaning rod: after it shot about 2 MOA with a scope). Both owners of the other 2 rifles were happy with much improved accuracy, but I was not able to test the rifles myself so I can't say how they shot. I've shortened the barrel on 3 rimfires, and all shot every bit as well at 20 yards with the shortened barrels as they did before (and one shot a lot better, I suppose because it originally had muzzle damage).
I sure don't suggest this if a rifle is worth spending a bit on! Take it to a gunsmith with a lathe and get it done right. However, for a rifle not worth putting money into, it is worth a try.
One tip I can add to help file the end square after cutting: bore a hole in 2 x 4 that will just take the muzzle. Split the 2 x 4 right though the hole with a saw, then clamp the muzzle in the board in a vise. It is quite easy to file the end square using this guide.
Now, I'm sure all the real gunsmiths will be cringing at the advice in this thread, and I would not disagree for a good rifle either!