PRINCIPLES OF GUNNERY. 53 
(and other guns similarly rifled), it is about 2J per cent. The increase 
of pressure in the bore tending to burst the gun, due to the resistance 
to the motion of rotation of the projectile, is very small: no sensible 
difference of pressure could be detected when the 10-in. gun was fired 
under similar conditions of charge and projectile, whether rifled or not 
*— i.e. } whether rotation was imparted to the projectile or not. 
The resistance due to friction with the B.L. Armstrong guns is con¬ 
siderable—-the friction extending over the whole of the surface of the 
projectile which is in contact with the bore; but in the service M.L. 
guns it principally arises from the friction between the edge of the 
studs and the driving surface of the grooves. With the M.L. guns, 
the projectiles are in most cases approximately centred; so that there 
is little or no friction between the plane surfaces of the studs and 
grooves. The coefficient of friction between the edge of the studs and 
the driving surface of the grooves may be taken to be about *17. 
In experimenting with powders of different shapes and densities, it 
has been considered desirable to assign a certain limit to the maximum 
admissible pressure in the powder-chamber, so as to avoid any undue 
strain on the material of the inner tube of the gun when repeatedly 
fired. The extreme limit is 25 tons on the square inch. The battering 
charges of the heavy Woolwich guns are regulated so as not to exceed 
this limit. With the 38 and 80-ton guns, it is considered desirable 
that the limit should not exceed 20 tons on the square inch. 
Resistance 
due to fric¬ 
tion. 
The maxi¬ 
mum limit 
of pressure 
allowable 
in the bore 
of heavy 
guns. 
