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.
The Birmingham people brown their barrels of inferior quality in the following way, to make them look equal to the best. They dissolve as much muriate of mercury as can be taken up in a dram-glassful of spirits of wine; this solution is mixed with one pint of water, or as much diluted as the person requires. A small quantity of the mixture is poured on a little whitening, and laid on the barrel with a sponge, rather lightly; as soon as dry, it is brushed off, and a fresh coat is laid on; and so on until the barrel is dark enough, which is generally about two days. The effect that the mercury has on every one of the joints of the fibres is wonderful: it never fails to make them, in two or three days at most, a beautiful brown; while the other parts, being harder, remain, comparatively speaking, quite light. The rust is killed by hot water, but after that, the barrels are suddenly immersed in cold water; which has the effect of heightening the brightness of both the colours. The appearance is beautiful, and equally as fine to the eye as stub-barrels browned in the same way; though this process is mostly used for the charcoal iron and the threepenny iron barrels. The only method in which there is no deception, is the smoke brown or stain; and, plainly speaking, this and no other is the reason the gun-makers condemn it. As the acid is decidedly weaker, and of course less liable to impart injury to the iron, no barrel can be browned by it, to look well and fine, but the best; or, in other words, none save those possessing steel in their composition.