PRINCIPLES OF GUNNERY. 
65 
be modified by the nature of powder, form of chamber, amount of air¬ 
space, &c., found advisable under the circumstances. 
There is a small loss of velocity due to the escape of gas through the 
vent. In the heaviest guns it becomes of importance to close the 
orifice of the vent, and so to prevent the wear of the bush caused 
by the action of the escaping* gas.* 
The muzzle velocity of a projectile is affected, under similar conditions, Twist of 
by the amount of work absorbed in giving it rotation ; but the work 
absorbed in giving rotation is so smallf (about '2 per cent, of the 
whole with the 16-pr. field gun), that practically there is very little 
difference in the muzzle velocity from this cause in service guns. 
From experiments made with the 8-in. M.L. gun, rifled (1) with 
increasing twist—0 to 1 in 40 calibres, (2) with uniform twist—1 in 
40 calibres, and fired under similar conditions of weight of projectile 
and charge of powder, it was found that in point of accuracy of fire 
the increasing twist had the advantage, but that the gun with uniform 
twist had a higher muzzle velocity and consequent length of range. J 
Table showing the Effect of the Employment of Uniform or Increasing Twist 
on Muzzle Velocity. 
Spiral, 
1 in 40 calibres. 
Charge of 
powder, R.L.Gr. 
Weight of 
projectile. 
Muzzle 
velocity. 
lbs. 
lbs. 
f.s. 
Increasing twist ... 
30 
179 
1303-3 
Uniform twist. 
30 
179 
1338-6 
The Ordnance Select Committee stated “that the supposed advan» 
tage of the increasing spiralis, that the projectile having, at the moment 
of leaving* its seat, only to move directly forward, gets away from the 
powder charge more readily than when—as in a gun with uniform 
spiral—it is forced simultaneously into rotatory motion. Somewhat 
more of the powder is therefore consumed before the shot moves in the 
latter than in the former case, and therefore the total force acting on 
the projectile during its passage through the bore is somewhat 
greater, and hence a somewhat greater muzzle velocity.” 
But this would hardly account for all the difference of velocity 
observed. The friction between the driving edges of the studs and 
the sides of the grooves in the gun is greater in the increasing than in 
the uniform twist for the same angle of spiral at the muzzle; for, on 
referring to the table at p. 41, giving the total pressure on studs (1) in 
uniform (2) in increasing twist,it will be seen that although the maximum 
pressure on the studs at any point in the bore is. less in the increasing 
than in the uniform twist, yet the aggregate of the pressures is greater in 
the increasing twist, and consequently the friction resulting therefrom is 
greater in the increasing than in the uniform twist. This friction absorbs 
some of the work done by the charge of powder, and a lower muzzle 
velocity results. The small loss of muzzle velocity in consequence of 
the employment of increasing twist, is, in the opinion of practical artil¬ 
lerists, more than counterbalanced by its advantages in other respects. 
# Vide “ Treatise on the Construction and Manufacture of Ordnance in the British Service,” p. 55. 
f Vide p. 40, note. 
X Vide “Proceedings, Ordnance Select Committee,” Yol. IV, p. 252. 
9 
