Just incase any one else thought it was a good idea... Turning the "bump" down on a lathe is going to end up really dangerous!!!
Here is why
A gun barrel is a tube if the outside is bulged so is the inside.
A lathe will indeed cut the "bump" back to the correct out side dimension but the "bump" still exists on the inside.
So what you end put with is a barrel that has a very thin spot in it!!!
This could and probably would explode as the structural integrity is now gone.
If your still not getting it think pull trigger and half the barrel goes flying down range. Or worse the barrel tears on one side and comes flying back at you.
Turn the "bump" down is a very stupid idea and a very good way to get you and others injured.
So in short if you don't know what your talking about keep your trap shut. Don't work on your fire arms if you don't know what your doing!!!
Please!