Originally Posted by slugthrower
The problem with back boring is that it usually prevents the use of slugs, the only reason I can imagine that causes this little drawback is a pressure spike when it hits the chokes and that the thinner walls at the beginning of the barrel can't take it, therefore you are advised, by manufacturers, to not shot slugs out of a back bored barrel.
Depending on the extent of backboring the slugs can actually hang up in the barrel as they'll wobble enough to turn sideways or angled in the barrel and jam. Then you've got an obstructed barrel. Slug number 2 can turn barrel into pipe bomb!
. Standard Accu-Mag Mossberg 835/935 barrels in particular are prone to this as they're overbored to near 10 ga. dimensions the entire length of the barrel. Dedicated slug barrels for those models are avialable though.