You do not have a 10 gauge barrel on your Mossberg unless you have had some extensive gunsmithing done. Overbore or backbore usually refers to barrels that have an inside diameter larger than the standard for that gauge. There have been, and are, factory examples produced by several manufacturers. Many gunsmiths offer the service. The merits are usually a source of debate. I got slightly increased velocities with some loads, and a marginal increase in pattern efficiency with several barrels I had backbored.
If you get too big, most plastic wads I have seen start running in to sealing issues. If you have too little or the wrong powder, you can lose a little velocity.