I'd say backbored or overbored are one and the same, and refers to something exceeding whatever is considered standard bore diameter.
As far as dimensionally speaking I don't think you can pin in down to one diameter. My SC gun is .750 but I've seen overbored barrels that measure up to .810 (12 ga that is.)

Last edited by Ken Nelson; 05/24/13 02:37 PM.
Dodging lions and wasting time.....