Is there a difference in the diameter of a 12 bore gun compared to a 12 gauge shotgun?

Obviously most shotguns have chokes but excluding the choke restriction.

Also I noted 450 said his RB's measured .735 while his bores measured .729, which would indicate a .006" size difference where the RB is BIGGER than the bore it is being fired through.... I thought .003 was about the max you could have a RB oversized... Will the lead actually deform that much to allow such an oversized "slug" to squeeze down and shoot out of a smaller-than-ball-diameter bore?