At a specified distance from the base of the shell a groove or band-score is turned for the rotation band. For projectiles under 7-inches calibre, pure copper is usually employed, but for larger calibre an alloy of 97½ per cent of pure copper and 2½ per cent of nickel is used and is annealed before banding. The rough bands are in a form of solid rings cut from drawn tubes or cylindrical castings, and must be carefully hammered into the score or preferably pressed in by hydraulic pressure and finally turned to proper size, shape, and finish.
Their use has been previously described and the improvements in armor-piercing shells hereinafter described are based upon a study of the stresses sustained by a projectile upon impact while rotating about its major axis at the high rotative velocity which the engaging of these bands with the rifling of the gun has imparted to the shell.
The following table compiled by the author gives the rotative velocities of various projectiles:
|
Calibre Inches |
Wt., lbs. Projectile |
Muz. Vel. Ft. Secs. | R.P.S. | R.P.M. |
Muz. Engy. Ft. Tons |
Type of Gun |
| 3 | 12 | 870 | 139 | 8,340 | 63 | Hotchkiss |
| 3.2 | 13.5 | 1,685 | 253 | 15,180 | 266 | Field '90 |
| 3.6 | 20 | 1,550 | 206 | 12,360 | 333 | " 1891 |
| 3.6 | 20 | 650 | 86 | 5,160 | 59 | Mortar 1890 |
| 5 | 45 | 1,830 | 176 | 9,560 | 1,045 | Siege 1890 |
| 7 | 105 | 1,085 | 76 | 4,560 | 853 | Howitzer '90 |
| 7 | 125 | 690 | 49 | 2,940 | 412 | Mortar '92 |
U.S. SEA-COAST LAND SERVICE GUNS
|
Calibre Inches |
Wt., lbs. Projectile |
Muz. Vel. Ft. Secs. | R.P.S. | R.P.M. |
Muz. Engy. Ft. Tons |
Type of Gun |
| 8 | 300 | 1,950 | 111 | 6,660 | 7,907 | 1888M |
| 10 | 575 | 1,975 | 95 | 5,700 | 15,548 | 1888M |
| 12 | 1,000 | 2,100 | 84 | 5,040 | 30,750 | 1902 |
| 16 | 2,370 | 1,975 | 59 | 3,540 | 64,084 | —— |
KRUPP GUNS
|
Calibre Inches |
Wt., lbs. Projectile |
Muz. Vel. Ft. Secs. | R.P.S. | R.P.M. |
Muz. Engy. Ft. Tons |
Type of Gun |
| 6 | —— | 2,600 | 192 | 15,520 | —— | —— |
| —— | —— | 3,000 | 222 | 13,320 | —— | —— |
| 8 | —— | 2,200 | 133 | 7,980 | —— | —— |
| 10 | —— | 2,250 | 108 | 6,480 | —— | —— |
| 12 | —— | 2,250 | 90 | 6,400 | —— | —— |
From the above table it will be noted that the R.P.M. are exceedingly high in some cases. Upon the impact of a shell with armor-plate the physical phenomena occur instantaneously and the resultant forces are so great that it is impossible to mechanically record their action. A study of the stresses in the shell can, however, be made on a theoretical basis.
In the first place, if the projectile were twenty calibres in length and of a material offering less resistance to torsional stress than steel and rotated at the high velocities indicated we would find that upon impact the torsion would be plainly evident as per the following: