slug a bore for cast bullets

[floydrturbo]

How do you slug or measure a rifle bore to determine the correct size cast bullet to use?

[John Traveler]

Slugging a bore is easy.

Start with a pure lead bullet or ball that is known to be slightly over bore groove diameter.  The swaged lead balls for black powder shooting are good, and so are some fishing sinkers; the kind that is split and you pinch to hold onto the line.  Your need the soft lead for an accurate measurement.

Clean the bore and leave it slightly oily to make slugging easier.

Using a mallet or plastic-tipped hammer and wood dowel rod or a cleaning rod that is close to bore size, press the ball into the muzzle.  Tap the ball down enough to start the dowel rod, and use the dowel rod to push the lead ball completely through the bore.  Catch it in a folded soft cloth or towel to prevent damage.

Do this a couple of times, and then use your micrometer to take MAXIMUM diameter measurements.  That is your bullet groove dieter.

HTH
John

[dla]

....then use your micrometer to take MAXIMUM diameter measurements.....

Slugging tells you the minimum diameter of your barrel, not the maximum. You will feel the tight/loose spots as you push the slug through the bore. On a Marlin 1895, you'll find a tight spot every place nomenclature was stamped or a dovetail broach-cut.

You'll likely see that the tight spot groove diameter is .457" and land at .450". Remember, that is the tight spot.

[marlinman93]

The method for slugging a bore chamges if you plan to shoot blackpowder. BP guns need to be slugged from the chamber end. A lead ball, or bullet is driven in just far enough to get a measurement of the forcing cone. It is the area between the bore and chamber that is utmost important for BP cartridges. This "ramp" area of the forcing cone is measured halfway between the chamber and the bore, and that is your bullet size for BP.
 I generally don't push a slug all the way through a bore for smokeless either. I drive it into the muzzle an inch or so, and then try another at the breech end about the same distance. I then measure both, and go with the larger of the two.
 I built a miniature slide hammer setup that I can put different sized lead balls onto. I then slide hammer them into each end, and slide hammer them back out. Then I can measure them. This setup allows me to tighten down on the lead ball, and expand it, so I can use it over again.