Keeping the cut at a right angle to the bore line is pretty critical. Much deviation will cause the shot cup to tip on it's way out. Probably result in a more open or patchy pattern. With slugs you may see some poorer grouping than would be the norm.
Buddy of mine did a good job on his, but he took longer than 15 minutes.
Does the gunshop include a new front sight?
"Hear the voices in my head, swear to God it sounds like
they're snoring." -Harvey Danger, "Flagpole Sitta