I'll put it this way. If you're any kind of halfway decent machinist, you already know the implications of fabbing up a part that is out of spec. It just won't work.
You also know that a few thousandths off, can be the difference from "it won't work" to "DANGEROUS". Have you ever seen a hydraulic line blow at the fitting? Very violent, and can F*CK YOU UP. We had a guy that would make fittings that were threaded REALLY short. Out of laziness? I do not know. We had a shitload of them blow & people get hurt.
If you can run a lathe, you can always pull the barrel from an action, and re-contour it. Chuck it up by the shank, & cut a nice taper. Have access to a Bridgeport? Get out your ball mills, time for you to cut fluting (assuming you can index the barrel)
Want to re-barrel a rifle? Get a nice barrel blank, turn down the shank, thread it, short-chamber it, and back it into the action. Finish headspace with the appropriate reamers & gauges.
If you have the knowledge to use such equipment, there are an INFINITE number of things you can do.
I'm sure John will chime in here. IIRC, he does some machine work too.
Sometimes you just have to Cowboy Up.