How would you slug the bore(measure) on a muzzle loader barrel?

Of course if you can remove the breech plug you just drive it through as with any breech loader. I've slugged a couple of RB ML's by drilling a hole in the ball to accept an 8/32 machine screw. The screw goes through the ball which is "sandwiched" between two smaller-than-bore washers. Then the end of a 5/16" aluminum rod was threaded to accept the ball, screw, washer assembly. The other end of the rod (OD) was threaded to accept a "knob" and a short length of steel tubing was placed on the rod to act as a "slide hammer" to bring the ball out of the bore. It's easier to make than it sounds and has worked for me...though there may be simpler ways. Drilling a hole through a pure lead round ball is not easy with a regular bit. I finally ended up using a course burr on a Dremel tool because a standard drill bit "grabs" the lead and destroys the whole thing. Of course grease everything up real good and go slow. Good luck.

Jon