First of all, "overbore" is an often misused and even more often misunderstood shorthand expression for the full term "over bore capacity." Bore capacity need not be a problem with cartridges that used to be over bore capacity with all the powders that were then available for handloading. With the slower powders available today, no practical cartridge is over bore capacity. (With the faster powders, of course, many cartridges are still over bore capacity for those powders.)
Bore capacity is easy enough to understand with a little thinking. The term refers to the capacity of the loaded case, with a certain weight of bullet. If a caseful of Powder A would produce excessively high pressure with the lightest bullet in that cartridge, that cartridge is over bore capacity for that powder. The cartridges that were traditonally labeled "over bore capacity" could not safely use a caseful of the slowest powders that were commonly available at the time -- notably IMR-4350 and H-4831.
Now, with IMR-7828, Ramshot Magnum, and H-50MG (and other slow powders), those once "over bore capacity" cartridges no longer are -- because they can safely handle case-filling charges of these ultra-slow powders at safe pressures.
A cartridge that's safe with a caseful of IMR-7828, however, is still badly over bore capacity with IMR-3031. A caseful of any too-fast powder would hoist pressures well above the safe limits. That's what "over bore capacity" means -- too much net case capacity for the powder to fill safely.
I've just been showing a friend the pieces of a once-fine custom rifle that another friend blew-up with a caseful of the wrong powder. A caseful of the right powder -- Ramshot Magnum or IMR-7828, for example -- produces only about 50,000 lb/sq in. in that cartridge. A caseful of the wrong (faster) powder that the rifle's owner loaded by careless oversight produced an estimated 120,000 lb/sq in., IIRC.
