I'm sure you mean "counter bored", not cross bored. A counter bore can be a chamfer or as the picture shows in an earlier post a larger diameter recess bored into the muzzle of a barrel, the reason for this is to prevent a burr or damage from something striking the end of the barrel pushing metal into the area of the barrel groove or land area. If you "dent" the end of the barrel and it has no place for the "bulge" of metal to go, it will interfere with the bullet when it exits the barrel thus deflecting it from it normal line of flight. (not hitting where you aim). Any gunsmith can add a chamfer to any barrel, but it needs to be done correctly as it must be properly centered and no "burr" can be left.