Backboring barrels.

It depends a lot on the gun, most of the newer guns (esp. target guns) have a fairly large bore dia. to begin with so backboring does little if anything to them. Usually the forcing cones are lengthened as well.

Older guns, say 10 years ago, for example a perazzi had pretty tight bores. This meant that the shot was more constricted at both the forcing cones down the barrel.

My understanding is that by opening the bore up you acheive better patterns (not necc. tighter but more even) because the shot is less deformed. Also I believe backboring does little if anything for recoil. Porting and lengthening your forcing cones will do more for that. Another reason I've seen it done is if you have a tight bore and a little (emphasis on little) barrel pitting backboring will generally remove the pitting restoring the barrel to new condition.

One thing to keep in mind is that if you currently have choke tubes you cannot have the gun backbored unless you also have new custom choke tubes made. About the only company I've heard that will do this is Briley, and no doubt for a high price. Also depending on how thick the barrel is it may or may not be able to be backbored. Some of the old over and unders had very thin barrels to begin with.