Basically you grind off a bit opposite the way that you want the bullet to go, I've seen it done to smooth ore shotguns for shooting round ball and foster slugs.
Actually doing it is not something I would recommend, not even slightly.
It's a dodgy half arsed way to screw a gun up with the hope of pulling a rabbit out of your hat.
Yes it can work, but it can backfire too and usually does if you try it yourself.

Better off playing with a load and bullets then then taking a grinder to a gun.