Does the barrel have to come off when changing the 458 Win Mag chamber to a Lott chamber?

Not if you are careful in your reaming operation. You can do it by hand you just have to be sure you do not cut into the chamber so far that you change the headspace. I've heard they have a 458 Lott reamer that will not cut the area for the belt and all you have to do is run it into the chamber till it hits the recess for the belt and quit but you don't have to worry about cutting the chamber too deep. I've done them both ways. I'm not sure which is easier.

the easiest/safest way is to dykem blue or red the belt recess and run it in about .275"; then start cutting the final .075" about .005" per pass until you see the dykem start to disappear. Can't see any more red/blue? Time to try a dummy round or the go/no-go gauges. My old fart gunsmith uses a piece of beige freezer tape as a gauge...he puts a layer on, taps around the edge with a piece of drill rod and tries it. When the bolt will almost close, he pulls the tape off and it generally is dead on. Has worked for him for 50+ years now.

Rich

YES
it should come off.

if you are doign conversions, it's best to order a conversion reamer... the belt is non-cutting

jeffe