Cutting down a shotgun barrel.

[evidrine]

Anyone had one cut down and threaded for choke tubes? My dad has a 20ga that I was using the other day and I have never been able to really hit anything with it. The more I looked at it I started to notice that the bead was slightly off center. Just enough to really throw you off when shooting at flying targets. Looking for a cost estimate on getting it cut down to fit removable choke tubes. Anyone know about how much I should expect to pay??