square. The edge of bore will be square all right when you finish the lapping operation to be described.
But first take a countersink reamer and countersink edge of bore to a depth of about 1/32 inch—this is just to get rid of surplus metal, and hasten the lapping.
Secure four brass balls measuring respectively 3/8, i/2, 5/8 and 3/4 inch in diameter. These may be secured from Ellfeldt Hardware & Machinist's Supply Company, at Kansas City, Missouri. I find that these are not regularly stocked by many large supply dealers. Drill about half way through each ball and tap for an 8 x 32 screw, which should be turned in tightly. Cut the head off an4 use the screw for a shank. Start with the smallest ball. Chuck it in a breast drill or brace, and with a small quantity of valve grinding compound lap the ball into the muzzle as shown in Figure 160, the lapping occurring on the extreme inner edge of bore. Continue with the 1/2 inch ball in the same manner, then with the two larger sizes until the inner half of barrel wall is well rounded. Round off the outer edge with a file to a corresponding shape, then polish the edge, first with No. 00 emery cloth, then crocus cloth held on the ball of thumb.
A muzzle lapped in this manner cannot be otherwise than square— for the simple reason that the surface of a sphere will not touch the
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