most other services, cartridge cases arc used for the smaller and medium guns only.
One of the most efficient obturators not liable to damage is the plastic device introduced by Colonel de Bangc of the French service and adopted by the French army and also by the British and other governments. It consists of a pad (fig. 52) made up of a strong annular-shaped canvas bag A, containing a mixture of asbestos fibre and mutton suet; the bag with its contents is placed in a properly formed die and subjected to hydraulic pressure by which it becomes hard and firm. The pad so made is then placed on the front of the breech screw B, and it is protected on its faces by disks C, C, of metallic tin or copper having steel wedge rings on the outer edges; the circumference of the complete pad and disks is
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Fig. 44.—English modern Breech Mechanism, for heavy and medium guns.
generally only slightly coned and fits into a corresponding seating formed at the breech end of the chamber, the canvas of the circumference of the pad being in immediate contact with the seat. In the English service the steep cone pattern (fig. 53) of de Bange obturator is used with mechanisms having the Welin screw. In front of the pad is placed a strong steel disk formed with a spindle, and called a mushroom head D, the spindle passing through the hole in the pad and through the breech screw, being secured in rear by a nut. The firing vent is generally drilled through the mushroom head and spindle and the part is then termed a "vent axial." On the gun being fired the gas exerts a great pressure on the mushroom head, which compresses the pad and squeezes it out on the circumference into close contact with the seating, thus forming a perfect gas seal. It is found that this apparently delicate arrangement will stand considerable ill-usage and act perfectly for an indefinite time, and, as it is easily replaced, it is regarded as one of the best and most reliable forms of obturator. In some countries the Freyre obturator is in use; this has a somewhat similar axial head to the de Bange, but the asbestos pad is replaced by a single steel wedge ring into which the axial head fits. On firing the gun the head is forced into the wedge ring and expands it against the seating in the gun.
One other means of obturation has to be considered, viz. metallic cartridge cases. These are made of a kind of brass; aluminium cases have been experimented with, but have not proved satisfactory. The case (fig. 54) acts on the same principle as the cup obturation and is extremely efficient for the purpose; moreover, they have certain advantages conducive to rapid firing when used for small guns. The idea has developed from the use of such cartridges in small arms, and larger cartridges of the same type were introduced for 3-pounder and 6-pounder guns by Hotchkiss and Nordenfelt about the year 1880 for the purpose of rapid firing against torpedo boats. Then in 1886 the Elswick Company produced a 36-pounder (soon converted to a 45-pounder} of 4·7-in. calibre with the powder charge contained in metallic cases, and about 1888 a 6-in. 100-pounder gun using similar cartridges. A special advantage of the cartridge case is that it contains the firing primer by which the charge is ignited and consequently renders the firing gear of the gun more simple; on the other hand, should a miss-fire occur the gun must be opened to replace the primer. This is a proceeding liable to produce an accident, unless a long enough time is allowed to elapse before attempting to open the breech; guns having de Bange obturators and firing tubes inserted after the breech is closed are therefore safer in this respect.
Some means of extracting the case after firing must be fitted to the gun; this is simple enough with small guns, but with those of heavy natures the extractor becomes a somewhat ponderous piece of gear.
Metallic cases of a short pattern have been tried for large calibre guns; although their action is quite efficient, they are difficult to handle, and if a case must be used it is preferable to employ a fairly long one. It was for this reason that in England up to 1898 it was considered that for guns above 6-in. calibre the de Bange obturation was the most advantageous. Since then the de Bange obturator has been employed in guns of 4-in. calibre and above, the cartridge case being retained only for 3-in. and smaller guns. Krupp, however, uses cartridge cases with all guns even up to 12-in. calibre, but this is undoubtedly due to the difficulties, which have already been noticed, attending the use of smokeless powder with the ordinary forms of obturation applicable to the wedge breech system. In the most modern Krupp 12-in. guns the charge is formed in two pieces; the piece forming the front portion of the charge is contained in a consumable envelope, while the rear portion is contained in a brass cartridge case, which forms the obturator, about 48 in. long.
It will be seen that such large and heavy cases add to the difficulties which occur in handling or stowing the ammunition of large calibre guns, and although the use of cartridge cases with small guns adds to their rapidity of firing this is not the case with heavy guns. It seems, therefore, that the balance of advantages is certainly in favour of the de Bange system, for all guns except those of small calibre. With ordinary field guns cartridge cases are now considered obligatory owing to their convenience in loading.
While the ordinary types of plastic obturators last for an indefinite time a cartridge case can be used for a limited number of rounds only, depending on the calibre of the gun; with field guns from ten to twenty rounds or even more may be fired from one case if care is taken to reform it after each round; with large guns they will not, of course, fire so many. Cartridge cases are an expensive addition to the ammunition, so that there should be no doubt about the advantages they offer before they are definitely adopted for heavy guns.
The rapidity with which modern guns can be fired and the enormous energy they develop is especially striking when one
