3-inch Boat-Gun (R. B. L.).
The breech-loaders with the exception of the bronze and steel boat-guns are as yet entirely converted from the muzzle-loaders of the Parrott type. In this conversion, the coiled hoop is removed, the cascabel is cut off to the powder-chamber, and the rear end of the bore as far forward as the trunnion is reamed out for the insertion of a steel tube, having a heavy screw-thread at its rear end by which it is secured in the casing. The old rifling is then carried through this tube, leaving the calibre unchanged. The coiled hoop is replaced by another of similar type, but covering a longer space. The breech mechanism is of the French type, with Broadwell steel gas-check fixed in its seat in the gun. The breech-block is provided with a steel nose-plate of the diameter of the face of the block, and having a copper ring countersunk in its forward face to form a bearing surface against the rear of the gas-check. The nose-plate is secured to the block by a long tenon which passes through the axis of the block and is secured by a nut at the rear. The vent is bored axially through this tenon, and is of two types, corresponding to date of fabrication, for preventing back-fire. In the first, a small steel plate covers the forward end of the vent, being loosely secured, so that on firing the flame escapes past the edges of the plate, but the back pressure holds the plate close over the vent; the action being precisely similar to that of an ordinary valve. In the other type, the forward part of the vent is reduced in size until just large enough to give free passage to the flame, giving it a needle-point, which causes the flame to pierce well into the cartridge. The back pressure is neutralized in this manner, as it reacts before the forward flame pressure is relieved. It is intended to so modify the firing arrangement as to use percussion primers, the gun-lock forming the mask to the vent. The crank is double, there being a vertical arm having at its upper extremity a crank and a pinion working in a rack on the face of the breech. The swinging tray for the breech-block is hinged at the right side of the breech, while at the left side is hinged a loading-tray having a sliding guard on it. The projectile being placed on this guard and the tray swung around to face the bore, on pushing forward the projectile the guard also enters the breech-block seat, bringing up against the gas-check, and thus guarding both the check and the screw-threads. In the boat-guns there is no loading-tray, the remainder of the breech mechanism being of the same type. Boat-guns are both of steel and bronze, being in both cases made of a single block.
Grooves.
The grooves are of two types. Those for the Parrott and converted guns are of the plain rectangular type, the rule for their number and size being that they shall be uneven in number to bring a land opposite a groove; lands and grooves shall be equal in width, and grooves for all calibres shall be the same depth—¹/₁₀₀ of an inch. Increasing twist. The Dahlgren type of groove is similar to the modified French groove ([see plate, page 187]), there being three only of a regular twist. These grooves are found only in the Dahlgren muzzle-loading bronze boat-guns.
Parrott Groove.
Dahlgren Groove.