Ditto for surplusrifle.com, you can go there and do a search for bore slugging.
Basically, it goes like this:
-You take a .25in wooden dowel rod (if you are slugging a .30 cal rifle)and cut it into 4 or 5 inch sections.
-Grease and lube the barrel like crazy.
-Take an appropriate sized fishing sinker, lead, of course.
-Tap it in from the muzzle and work your way down, using a rubber mallet to tap in the dowel sections, one at a time, until the slug comes out of the chamber end.
-Measure the marks the grooves and lands left on the lead slug with a vernier caliper, the biggest measurement is basically your bore diameter.
This works great with 2, 4, and 6 grooved rifles (or any even number), but 5 grooved rifles are a bit trickier to measure.