The percussioning of a gun (as the fitting-in of nipple, boring breeches, filing cocks, &c., is termed,) is also a mechanical operation, requiring workmen of the very best abilities. The desideratum to be obtained is nearly a direct communication into the barrel, and an absence of unnecessary angles, antechambers, &c.; therefore it is needful that, in a double gun, the nipples should be inserted as near the centre of the breeches as they can be conveniently placed, with the nipples standing, not upright, but at an angle of 45°; so that the blow of the cock shall be in or as near a line with the imaginary upright of the nipple as possible. The various plans of copper cap, copper tubes, and I know not how many other devices, will be discussed under the head of guns and shooting. Finishing the stock, polishing, engraving, hardening, &c., strictly speaking, involve no science of consequence, and as such it is scarcely necessary to occupy the attention of the reader respecting them. The best method of staining barrels is by the following recipe: but one material fact must not be overlooked. A considerable difficulty exists in staining barrels all steel; in such a case, therefore, the acid should not be so much diluted. 1 oz. muriate tincture of steel. 1 oz. spirits of wine. 1/4 oz. muriate of mercury. 1/4 oz. strong nitric acid. 1/8 oz. blue stone. 1 quart of water. These are to be well mixed, and allowed to stand a month, to amalgamate. After the oil or grease has been removed from the barrels by lime, the mixture is laid on lightly with a sponge every two hours. It should be scratched off with a steel-wire brush night and morning, until the barrels are dark enough; and then the acid is destroyed by pouring on the barrels boiling water, and continuing to rub them till nearly cool.