Bore and weight limited.
Premising, that when Mr. Whitworth was solicited by the late honored Lord Hardinge to render the aid of his mechanical genius to the improvement or perfecting a military weapon, he was restricted as to length of barrel, viz., 3 feet 3-in., and weight of bullet, ·530 grains. We shall now proceed and use Mr. Whitworth’s words.
Consideration for curve.
“Having noticed the form (hexagonal) of the interior which provides the best rifling surfaces, the next thing to be considered is the proper curve which rifled barrels ought to possess, in order to give the projectile the necessary degree of rotation.”
Hexagonal form admits of quick turn.
“With the hexagonal barrel, I use much quicker turn and can fire projectiles of any required length, as with the quickest that may be desirable they do not ‘strip.’ I made a short barrel with one turn in the inch (simply to try the effect of an extreme velocity of rotation) and found that I could fire from it mechanically—fitting projectiles made of an alloy of lead and tin, with a charge of 35 grains of powder they penetrated through seven inches of elm planks.”
Degree of spiral fixed.
After many experiments, in order to determine the diameter for the bore and degree of spirality, Mr. Whitworth adds: “For an ordinary military barrel, 39 inches long, Diameter of bore determined.I proposed a ·45-inch bore, with one turn in 20 inches, which is in my opinion the best for this length. The rotation is sufficient with a bullet of the requisite specific gravity, for a range of 2000 yards.” Under these conditions the projectiles on leaving the gun would be about two and a half diameters of the bore in length. “The gun responds to every increase of charge, by firing with lower elevation, from the service charge of 70 grains up to 120 grains; this latter charge is the largest that can be effectively consumed, and the recoil then becomes more than the shoulder can conveniently bear with the weight of the service musket.
Advocates of slow turn.
“The advocates of the slow turn of one in 6 feet 6 inches, consider that a quick turn causes so much friction as to impede the progress of the ball to an injurious and sometimes dangerous degree, and to produce loss of elevation and range; but my experiments show the contrary to be the case. Effects of quick turn.The effect of too quick a turn, as to friction, is felt in the greatest degree when the projectile has attained its highest velocity in the barrel, that is at the muzzle, and is felt in the least degree when the projectile is beginning to move, at the breech. The great strain put upon a gun at the instant of explosion is due, not to the resistance of friction, but to the vis inertiæ of the projectile which has to be overcome. In a long barrel, with an extremely quick turn, the resistance offered to the progress of the projectile is very great at the muzzle, and although moderate charges give good results, the rifle will not respond to increased charges by giving a better elevation. If the barrel be cut shorter, an increase of charge then lowers the elevation.”