Hi Slash
Before you whack the end of the barrel off, check to see if making it a fraction of an inch longer will allow for one more cartridge.
How to do this easily? Cut the mag tube about one inch too long first and try the actual cartridges you will use in the rifle.
The first cartridge you push into the mag should be made longer than all the others to give you a little safety factor. Maybe .25 inch. Cut the tube in .125 inch increments. It will probably take several cut-n-tries.
After the mag tube is cut, you can cut the barrel, just be sure to leave enough extra metal for crowning as well as remaining legal.
If you crown in the lathe, be sure the crown is centered on the bore, not the barrel outside.
A light internal chamfer cut after facing the barrel will let you see if the chamfer is centered on the bore and the crown will be true. If not, you will have to make a plug for the bore and indicate off of that to find the bore center. Then recut the crown.
Quite frankly, you can face the barrel on a table saw with a sanding disk and make the chamfer with a round head screw and valve grinding compound. The difference in accuracy between the two methods will never show up with open sights.