The length of friction depends entirely on the length of the barrel. Long barrels require more than short, though the latter require it in a greater degree. A mode of creating friction, much practised by those who are ignorant of the true method, is to bore the barrels as rough and as full of rings as possible. These rings are often taken for flaws; though that may be ascertained by noticing whether or not they have the same inclination as the twist, and whether or not they are at the jointing of a spiral. If they be not, the chance is that the barrel is ring-bored, as it is termed. This roughness, however, answers the same as friction by relief; but barrels thus roughened are very liable to lead, and become foul. While the well-bored barrel will fire forty shots as well as twenty, these cannot be fired more than twenty times with safety and effect.
Each of the barrels in the table below, if 3-16ths thick at the breech, is equal to the pressure stated. The resistance of a charge of shot of one ounce we find to be more than before stated; and the additional increase of explosive force obtained at the moment of ignition, requires the amount to be much greater in computation, therefore, we may safely take a pressure of 1,700 pounds to the inch of tube. The reader will perceive, on reference to the following table, that with the tube filled with powder for an inch in length, which is a small charge, the explosive force will be equal to 40,000 pounds, or nearly 1,700 pounds to the inch.
| Pressure of charge. | Surplus strength. | ||||
|---|---|---|---|---|---|
| lbs. | lbs. | lbs. | |||
| Laminated and other steel barrels are equal to a pressure of | 6,022 | 1,700 | 4,329 | ||
| Wire twist | 5,019 | 1⁄2 | 1,700 | 3,319 | 1⁄2 |
| New stub twist mixture | 5,555 | 1,700 | 3,855 | ||
| Old stub twist | 4,818 | 1,700 | 3,118 | ||
| Charcoal iron | 4,526 | 1,700 | 2,826 | ||
| Threepenny skelp iron | 3,841 | 1,700 | 2,141 | ||
| Damascus iron | 3,292 | 1,700 | 1,592 | ||
| Fancy twisted steel | 3,134 | 1,700 | 1,434 | ||
| Twopenny skelp iron | 2,840 | 1,700 | 1,140 | ||
If the charge he increased to one ounce and a half, the length it occupies, and the lateral pressure by the jamming, will create an additional pressure in proportion, or near 2,550 pounds, as under:—
| Pressure of 11⁄2 oz. shot. | Surplus strength. | ||||
|---|---|---|---|---|---|
| lbs. | lbs. | lbs. | |||
| Laminated and other steel barrels are equal to a pressure of | 6,022 | 2,550 | 3,472 | ||
| Wire twist barrel | 5,019 | 1⁄2 | 2,550 | 2,469 | 1⁄2 |
| New stub twist mixture | 5,555 | 2,550 | 3,005 | ||
| Old stub twist | 4,818 | 2,550 | 2,268 | ||
| Charcoal iron | 4,526 | 2,550 | 1,976 | ||
| Threepenny skelp iron | 3,841 | 2,550 | 1,291 | ||
| Damascus iron | 3,292 | 2,550 | 742 | ||
| Fancy twisted steel | 3,134 | 2,550 | 584 | ||
| Twopenny skelp iron | 2,840 | 2,550 | 290 | ||
A charge of shot two ounces weight will be greater in pressure than barrels of these dimensions are equal to restrain, and, consequently, no barrels should be charged to this extent at any time; but inferior barrels, as a matter of certainty, are sure to give way if so loaded.
| Pressure of 2 oz. shot. | Surplus. | ||||
|---|---|---|---|---|---|
| lbs. | lbs. | lbs. | |||
| Laminated barrels, &c. | 6,022 | 3,400 | 2,622 | ||
| Wire twist barrels | 5,029 | 1⁄2 | 3,400 | 1,619 | 1⁄2 |
| New stub twist mixture | 5,555 | 3,400 | 2,155 | ||
| Old stub twist | 4,818 | 3,400 | 1,418 | ||
| Charcoal iron | 4,526 | 3,400 | 1,126 | ||
| Threepenny skelp iron | 3,841 | 3,400 | 441 | ||
| Damascus iron | 3,292 | 3,400 | |||
| Fancy steel barrels | 3,134 | 3,400 | |||
| Twopenny skelp iron | 2,840 | 3,400 | |||
The foregoing tables show clearly the danger of persevering in using heavy charges of shot; for it must be borne in mind that accidental circumstances will increase this pressure, and never can act so as to lessen it: a foul gun, or a variety of other circumstances, being sure to increase the danger.