15]
The BOTTOM OF THE BORE (to facilitate sponging) is a plane perpendicular to the axis, united with the sides (in profile) by an arc of a circle the radius of which is one-fourth of the diameter of the bore at the bottom. In the columbiads, the heavy sea-coast mortars, stone mortar, and eprouvette, the bottom of the bore is hemispherical.
The MUZZLE, or mouth of the bore, is chamfered to a depth of 0.15 inch to 0.5 inch (varying with the size of the bore), in order to prevent abrasion, and to facilitate loading.
The TRUE WINDAGE is the difference between the true diameters of the bore and of the ball.
27. What is the vent?
The aperture through which fire is communicated to the charge.
28. What is to be observed in reference to the diameter of the vent?
It should be as small as the use of the priming wire and tube will allow.
29. Why?
As the velocity of the gases arising from the combustion of the powder is extremely great, a large amount escapes through the vent, which contributes nothing to the velocity of the projectile. It therefore follows, that the effect produced by a given charge will diminish as the diameter of the vent increases. Besides, on account of the increase of power in the current that escapes from them, large vents are more rapidly injured than small ones.