Ain't gonna happen with a drill. Drill bits walk too much.
To do it propertly you need a lathe. The bore has to be lined up perfectly, not the barrel, the bore. Often they're not the same.
If I was going to do it, I'd remove the barrel from the action, using a 4 jaw chuck line up the bore, then counter bore the muzzle.
Before you do anything, shoot it to see if it really needs counter bored. You might find out it shoots fine just like it is.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071