First lets differentiate between chamfering and deburring. A chamfer is an angle added to the inside of a case neck, also on the outside. It "breaks the sharp, square end of the neck. Deburring, is removing the burr created when trimming brass cases. The cutters used to trim brass, have a very small engagement angle to prevent them from grabbing or diving into the brass. Because of that, they don't cut very clean, raising a burr.
It happens on the inside and outside of the brass at the end of the neck. Failure to remove it,(the burr), would cause lots of problems. Bullets being seated would get damaged by the burr, and some brass shavings could end up inside the case. The outside of the neck would have that burr forced back by the sizer die, it could smear onto the outside of the neck.
For handgun brass I do NOT chamfer them either inside OR outside. Since I don't trim semi-auto brass, I don't deburr either.
I did once trim and deburr revolver brass. It was when I was active in handgun silhouette shooting. It's a long range shooting sport, so consistency was paramount. However, I did not chamfer them. You want the inside edge of the case mouth to have a square edge when crimping, it bites into the bullet cannelure better. Shooting a 44 revolver @ 200 yds, demands the utmost in consistency. And max charges of slow burning powder demands a heavy crimp.
