Accuracy with a roundball rifle is based on a paradox; an underside ball is spun by rifling that it never touches! The secret, if there is one, lies in the cloth patch. It functions as a gas seal and, theoretically at least, completely seals the bore. It also transfers to the ball the grip and the spin of the rifling. To perform its duties, the cloth patch should fill the grooves of the rifle. In other words, it should be under considerable compression not only where it contacts the lands, but also at the bottom of each groove. A tight fitting combination of patch and ball is an absolute must for accuracy and considerable cloth must be packed into each groove to completely seal the bore.
When a perfectly patched roundball is driven through a barrel, it will show cloth marks completely around its circumference, like the ball on the right. These marks will be light where the ball and patch touched the grooves and heavy where they touched the lands.
Your selection of the proper cloth patching should he based on an understanding of the relationship between the bore of your rifle and the roundball which will be used. Ball diameter must be less than that of the bore and the cloth must not only fill the grooves but also allow a tight sliding fit between the lands and the patched ball. Follow the suggestions of your rifle's manufacturer concerning projectile diameter. Several companies make specific diameters available either with the purchase of the gun or as a component in a valuable accessory kit. Most other guns will have standardized bore sizes and will be suited for one of the standard roundball diameters such as .490", .495", .530" or .535". To make your final judgment on ball and patch you must measure the bore.
The best way to measure the bore of our rifle is with a soft lead slug which has the rifling engraved on its diameter. Remove the barrel from the stock. Slide a brass rod, which is about 12" long and just under bore diameter (about 3/8" or 7/16"), into the barrel. Start an oversized slug into the muzzle and drive it into the barrel approximately 2" using a brass punch. Now tip the muzzle end of the barrel downwards so that the brass rod slides into the slug. Repeat this procedure several times until the rod drives the slug from the barrel. The engraved slug will have a perfect print of your bore's dimension. Now, with a micrometer, measure both the bore and groove diameter. Here's a sample:
We know the ball must be smaller than the bore diameter so let's select a .498" diameter ball as the best choice for this bore.
With the ball adequately undersize (.005") to fit into the bore we now determine the needed patching thickness to seal the grooves.
Groove .526" Ball -.498"
Now divide the difference by two and the minimum patching thickness is determined: .014". Remember, there is a thickness of patching on each side of the ball and the difference between the ball and groove diameters must be halved to determine the thickness of cloth needed. Usually it is better to buy cloth that is several thousandths of an inch thicker since the lubricated material will compress upon loading.
Now that you're on the way-keep experimenting with your rifle. Vary the powder charge, cleaning technique, patching or whatever. That's part of the fun of muzzle-loading. But remember to vary only one condition at a time so you can easily keep track of cause and effect.
Lyman suggests that only natural-fiber cloth be used for patching; not synthetics or natural/synthetic blends. The heat of ignition can melt some synthetics resulting in inaccuracy and deposits in your bore. Furthermore, Lyman suggests that plastic cups or patching systems not be used since there can be inadvertent misuse which results in unsafe shooting conditions. Stick with the traditional cloth patch.
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