I have a lathe and many years ago I made a .410 insert for my Stevens single 12 ga. Tolerances were not a major issue but it worked. Only issue was the insert wanting to eject and the price of ammo. I ran across it a year ago and thought about making one for .45acp. I had a piece of SPS steel that I was going to make a motorcycle axle out of years ago that never got used. This one I made with close tolerances and finished the bore at .450. The lathe that I used was an old Atlas that you had to change gears to change feed rate. With spacers and such I got affected rate of. 16" per revolution. I used the back gears as well. I made in a boring bar and silver soldered a tool bit to the end and the ground it on the lathe to a .454 diameter and about .040 wide. I did one pass through the bore and nothing broke so I quit. One groove is a bit deeper than the other and one a bit wider probably due to the difference in the cutter which I managed to drop and break one of the cutters.
I took it up moose hunting and we had a bit of fun shooting firewood logs and the odd spruce tree. Although the only sight was the shotgun front bead it was fairly accurate but of no practical use.
Just something to keep me busy in the shop