
177Weapon Design Practice
Note here that F
P
only acts on the bore diameter, whereas F
R
acts on the breech face diam-
eter, normally larger than the bore. The resistance pressure is estimated as follows:
For rifled bores: (tan )
R
F
P
P
r
r
≈+
T
F
(5.18)
The riing force is
F
k
F
T
2
P
tan =
α
(5.19)
where
k is the projectile’s radius of gyration (I
zz
= wk
2
in terms of axial moment of inertia and
mass)
μ is the coefcient of friction, and α is the riing angle
Our object is to nd F
P
.
From the Lagrange approximation for the pressure gradient we know that
pp
c
BS
1
(5.20)
In terms of forces, this may be written as
FF
wc