2 
discharged, the gun is expanded by the internal pressure and as the driving-band 
has become adapted to the shape and dimensions of the bore at the starting of the 
projectile, it follows that when the latter has travelled a certain distance or, in 
other words, when the dilatation of the gun becomes appreciable, the driving-band 
cannot keep up with the increased diameter of the bore and must, therefore, 
necessarily fail to fulfil one of the chief objects for which it is intended, allowing, 
as it does, the gas to rush past, causing the fatal erosions. 
It may also happen that the diameter between two opposite grooves may present 
the maximum tolerated length in manufacture, whilst the driving-band might 
only be of the minimum tolerated gauge or, in other words, that the interior 
diameter between the grooves is unequal to the exterior diameter of the band. 
When this happens the-escape of gas, on expansion of the bore, would attain a 
much higher degree. 
We, therefore, see that in spite of the most scrupulous care in gun manufacture, 
gas escape may even happen during the first rounds fired from a new gun whilst 
guns which are worn by firing would, of course, be more liable to this result. 
A careful inspection of bores in which erosion has been produced, demonstrates 
beyond dispute the truth of the foregoing remarks since it is noticed that although 
such erosion may commence at the beginning of the rifling, the cavities are 
shallower and less important at this point than a little further on towards the 
muzzle. This is explained by the fact that the first volume of gas, developed 
from the charge, overcomes the inertia of the projectile and starts it off with a 
weak pressure whereby the velocity of the escaping gas is only slight during the 
first moment and the effects produced are proportionately limited. As the pressure 
increases and reaches its maximum limit, which happens when the projectile has 
travelled a certain distance in the bore, the gas escapes with the maximum velocity 
producing the greatest amount of damage. Likewise, as the maximum pressure 
becomes exhausted, the energy of the gas gradually falls and the erosion is reduced- 
and totally disappears close to the muzzle. 
It is also noticed that erosion is more generally to be found in the grooves than 
along the lands and the reason of this is obvious : for the lands of the rifling, 
becoming imbedded in the driving-band, not only hermetically seal the passage of 
the gas, but are also further protected by the tongue of metal which each land 
displaces in the band. 
The shape presented by erosion cavities is that which would be offered by an 
impression of a series of drops of water linked to one another or grease flakes 
hanging from a candle and their disposition in the grooves is such, that their 
Wildest extremity is next to the breech. This shape might be due to the irregular 
action of the gas which, having penetrated between the projectile and the bore, 
fails to find an outlet for its entire volume and is therefore repulsed and forced to 
spread and expand between the cannelures of the driving-band and the grooves of 
the rifling, thus forming strong currents in the already initiated erosion. What¬ 
ever may be the cause establishing their shape, it is a certain fact that, whether 
isolated or in groups, these cavities are united by a continuous channel of irregular 
dimensions whence it may be assumed that their formation must chiefly be 
attributable to gas escape between the driving-band and the sides of the bore, 
since, were they due to other causes, such as high temperature or chemical 
affinity between the products of combustion and the metal of the gun, they would 
undoubtedly be of the same size along the whole portion of the bore which is 
subjected to the same temperature and pressure, which, as has been stated, is not 
the case. 
Doubtless even these last mentioned causes have a direct influence in the pro¬ 
motion of erosion and help to augment if, but the principal cause lies in the 
