For producing destructive effect among troops, a third kind of projectile is employed. It is called shrapnel, and it consists of a thin shell, holding a little powder and a large quantity of bullets. The powder is ignited by a fuse, which is set to act during flight, or on graze, when the shell is nearing the object. The explosion bursts the shell open, and liberates the bullets, which fly forward, actuated by the velocity of the shell at the moment of bursting. Hence, to render the bullets effective, a considerable remaining velocity is requisite. The gun must therefore take a large powder charge, while, as the shell has to hold as many bullets as possible, the bore must be large enough to take a short projectile of the given weight. Thus, the proportions of the shrapnel gun will be intermediate between those of the armor-piercing gun and the shell gun. There are certain axioms known from experience, which should be mentioned here. First, the length of the powder chamber should not be more than three and a half or four times its diameter, if it can possibly be avoided, because, with longer charges, the inflamed powder gas is apt to acquire rapid motion, and to set up violent local pressures. Next, the strength of a heavy gun, as reckoned on the principle of all the metal being sound and well in bearing, should not be less than about four times the strain expected. Again, though there are several opinions as to the best weight of shot for armor piercing, in proportion to diameter, yet among the most advanced gun-makers, there is a growing tendency toward increased weight. The value of w/d³, that is, the weight in pounds divided by the cube of the diameter in inches, as this question is termed, is in the hands of the Ordnance Committee, and it is to be confidently hoped that efforts will shortly be made to arrive at a solution. In the meantime, from about 0.45 to 0.5 appears to be a fairly satisfactory value, and is adopted for the present. Lastly, it may be broadly stated, that with suitable powders, a charge of one-third the weight of the shot demands for most profitable use a length of bore equal to about twenty-six calibers; a charge equal to half the weight of the shot should be accommodated with a bore of about thirty calibers; while a charge of two-thirds the weight of the shot will be best suited by a bore thirty-five calibers long. Of course, in each case, greater length of bore will give increased velocity, but it will be gained at the expense of additional weight, which can be better utilized elsewhere in the gun.