This board's sponsor, Beartooth Bullets, sells slugging supplies. Basically, you clean the bore completely, run a lightly oiled patch through it and on the slug. The slug can be a bullet specially cast of pure lead, a pure lead ball that's a little bigger than groove diameter, or a pure lead fishing sinker (often these are not lead but some harder alloy that doesn't work, but Beartooth sells the right kind).
Pure lead, for slugging purposes, is inelastic, so it reflects the diameter of whatever bore you put it through, at least to the nearest ten-thousandth of an inch that a micrometer can measure. Harder alloys than pure lead, like regular cast bullet alloy, are partially elastic. They spring out against the bore by as much as half a thousandth, making it impossible to tell a modest constriction from a wide spot or to get an accurate measurement. Pure lead is required. If you want to cast your own slugging bullets, stick-on wheel weights typically are pure enough. A micrometer with ten-thousandths resolution is generally required, as calipers can be off a half or a thousandth pretty easily. Enco has them in the $20 range.
You tap the lightly oiled slug into your lightly oiled muzzle or throat with a brass or wood dowel a little smaller than bore diameter and with a hammer. You can push the slug out from the other end with a cleaning rod, then measure it to learn muzzle or throat diameters. You can also push it slowly all the way through the bore. This lets you feel for constrictions, where the lead is just slightly harder to push through or is felt to be rubbing more than in the rest of the bore.
Measuring a slug that has gone all the way through the bore tells you the narrowest diameter of your bore, which you then can compare to the throat and muzzle. Nothing should be smaller than the muzzle or lead bullet accuracy will be compromised. Firelapping or hand lapping clears constrictions.
I certainly can't improve on the advice of unclenick, but i can assure you it is very easy to do. I found the right lead sinkers and with some oil and not to much effort i had some nice slugs for cast bullet info !!
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