36 
PRINCIPLES OP GUNNERY. 
Examina¬ 
tion of 
recovered 
projectiles 
for groove 
marks on 
body and 
studs. 
In several Woolwich, guns the twist is zero at the breech end of the 
rifling; so that in this case 
tan a = — — 0, or — = 0. 
m m 
So that from equation (A) the twist of rifling at a point in the bore at 
h inches from the muzzle is 
one turn in 
n 
= — — - - calibres; 
l — h 
(C) 
and from equation ( B ) the linear distance of rotation of the exterior of 
the projectile at a distance of h inches down the bore is 
It is often important, when examining projectiles which have been 
recovered after firing, to note the marks of the grooves on their bodies 
and studs, in order to determine the exact position in the bore of the 
gun where these marks were impressed. 
For example : an 11-in. common shell, after having been fired, was 
recovered and examined for marks of the grooves on the driving edges 
of the studs. The impression on the front stud was due to a twist of 
one turn in 385 ins., or one turn in 35 calibres—the same as at the 
muzzle of the gun; showing that the front stud was in bearing at the 
muzzle. The impression on the rear stud was due to a twist of one 
turn in 592 ins., or one turn in 53’8 calibres; and the impression of 
the groove marks on the body of the shell (owing to contact of the 
body of the shell with the gun, from defective centring), was due to a 
twist of one turn in 1584 ins., or one turn in 144 calibres. 
From these data it can be ascertained at what distance from the 
muzzle the rear stud ceased to rotate the projectile, and the front stud 
came into bearing; and also at what distance from the muzzle the 
shell must have been when there was contact between the body of the 
shell and the bore of the gun. 
The length of rifling in the 11-in. M.L. gun is 119 ins., and the 
twist starts from zero at the breech to one turn in 35 calibres at the 
muzzle. 
The amount of twist shown at the last contact of the rear stud with 
the side of the groove was one turn in 53*8 calibres. 
From formula ( C ) 
(where l — 119 and n — 35), 
or 53*87* = 2237% 
or It = 41'5 ins. from the muzzle; 
which shows that the front stud came into bearing at 41*5 ins. from 
the muzzle. 
