TREATISE
ON THE
CONSTRUCTION AND MANUFACTURE
OF
ORDNANCE IN THE BRITISH SERVICE.
PREPARED IN THE ROYAL GUN FACTORY.
PRINTED BY ORDER OF THE SECRETARY OF STATE FOR WAR.
APRIL 1877.
pp. 51-54
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VENT PLACEMENT The position of the vent in the gun may either be what is usually called axial or central, i.e., when the vent channel runs through the axis of the breech and strikes the cartridge in the centre, at the bottom of the bore, as with our R.B.L. guns; or it may be such as to strike the chamber at an angle, or perpendicularly to the axis near the bottom or elsewhere, as is done in our S.B. and R.M.L. ordnance.
For convenience the vent generally opens at the top of the gun; but in 10-inch guns and upwards, where the size of the gun renders this position awkward, the bush is placed at an angle of 45° with the perpendicular, and the vent hole will therefore be at the top right side in such guns for broadside and garrison service, while in turret guns it is sometimes placed on the left side and sometimes on the right as convenient.
When powders of comparatively small grain are employed, the rate of ignition of the charge, and so the maximum pressure on the shot and gun, is influenced considerably by the point at which ignition first takes place, i.e., by the position of the vent.
This is more particularly the case when large charges of considerable length are employed.
From experiments made in 1863 with large charges of R.L.G., the powder then employed with heavy guns, it appeared that when the cartridge was ignited at a distance of 4/10ths of its length from the bottom of the bore that the best results were obtained as to velocity imparted to the projectiles; it was therefore settled that heavy guns were to be vented so that the bush should strike the bore at that distance from the bottom.
In 1863 General (then Colonel) Lefroy submitted that "the time had come when it was desirable to consider whether the habitual practice of igniting the charge at the back end is favourable for developing the greatest force of the charge with the least strain on the gun, the elongated charges coming into use with the heavy shot and powder of larger grain than formerly having altered the old conditions."
Experiments were then sanctioned to be carried out in a bronze gun of 6".5 calibre bored up to 6".9. It was vented in six different positions, viz.:
No.1. Came out exactly at the base of the charge.
" 2. Half-way between that and the service vent.
" 3. Service vent
" 4, 5, and 6, respectively, 1.38, 2.76, and 4.25 in front of the true vent.
With 14 Ibs. charge R.L.G. powder and 42-lb. shot, the following mean initial velocities were obtained from the above-mentioned vents:
1st Experiment. 2nd Experiment.
Feet. Feet.
No. 1 vent - - 1,773 - -
2 - - 1,782 - -
3 - - 1,789 - - 1,794
4 - - 1,816 - - 1,836
5 - - 1,861 - - 1,895
6 - - 1,811 - - 1,860
It is observed that the velocity increased as far as the 5th vent, the greatest increase being from the 4th to the 5th, at which point the mean initial velocity was increased 72 feet per second above what had been obtained when using the service or No. 3 vent. In proceeding to No. 6 vent, the velocity was suddenly diminished 51 feet per second." These experiments were repeated, but the difference between Nos. 3 and 5 was still greater, being now 100 feet per second. " Taking the mean between the two results, viz., an increase of 86.5, it is equivalent to an increase of the charge from 14 Ibs. to 15 1/2 Ibs., and in the opinion of the committee proves conclusively that the vents of guns using heavy charges should be bored to as to strike the cartridge in the position of No. 5 vent, that is to say, at about 4/10ths of its length from the end of the gun, or slightly behind the centre of the cartridge. The committee recommend that the position of the vent in all future wrought iron M.L. guns, be such as to fulfill this condition, and that the vent be drilled vertically as already adopted by Sir W. Armstrong in all his larger guns.
Both of these recommendations were approved 14/4/64, as the experiments had proved that the ignition of the cartridge at the point above-named "realizes the greatest projectile force which can be produced by a given charge."
No account was taken in these experiments of the pressure in the bore, which has since been proved to be much greater when charges of R.L.G. powder are fired with the forward vent.
