Unlike other brands, which offer backbored barrels, Mossberg's overbore
design spans the entire length of the barrel, providing a gas cushion for the shot charge.
Cartridges have been accused of being "over-bore capacity", or simply "overbore", as though the quantity being described were inherent in the cartridge itself, which it isn't.
On the other hand, a case with so much capacity that the delta point is easily attainable even with the slowest propellants available today may be classed as "overbore".
However, many non-belted cases, such as the .17 Rem., .243 wildcats on '06-sized cases, .25-06, and others are right on the line, so "overbore" is not a disease solely of magnums.