I have a Rockwell 21-100 mill with a head that rotates side to side.

The head on it's side will run into the table before I get the barrel in the vise on the table high enough to get the spindle parallel with the barrel.

I could cut perpendicular to the barrel, and then file out the uncut corners by hand.

I am looking at a Ruger #1 re barrel right now.

TIA