Hutch,
Slugging the bore is no problem,just cut open a 16ga #2 buckshot shell, for the 28 cal.shot. Carefully tap one of them into the muzzle of the barrel(remove it from the gun).Use a small hammer until it is almost into the barrel,then use a punch(be sure it will fit into the barrel)to drive it onto the barrel about an inch.Push the slug out with a rod,from the chamber end. Then, its just a matter of measuring it.This shouldn't be a problem as long as there is an even number of grooves.
Mike