=Separate Ammunition.—Obturation.= The 155-mm Filloux gun and 155-mm howitzer use separate ammunition. In such guns there must be provided some form of a gas check which will prevent the powder gases from rushing to the rear into the threaded portion of the breechblock, as this would soon erode the thread sectors and render the gun useless beside losing a large amount of pressure in the bore. The device used as a gas check is called an obturator. There are two systems of obturation in use, named after their inventors: The DeBange and the Freyre. The former is used in the 155’s. It consists of a steel mushroom head closing the rear of the powder chamber, the spindle of which passes through a central hole in the breechblock. Between the mushroom head and the face of the breechblock is a pad of asbestos, paraffine and tallow, pressed into shape by a hydraulic press and covered by canvas or asbestos wirecloth. Split rings having hardened outer surfaces are fitted, one just behind the mushroom head and one just in front of the face of the breechblock. Their diameter is slightly greater in the free state than the conical surface of the bore where they bear when the breech is closed so that they always close the rear of the powder chamber. The pressure of the powder gases forces the mushroom head to the rear and this compresses the asbestos pad which in turn forces the split rings to bear with greater force against the walls of the powder chamber thus securely closing the rear opening of the powder chamber. For more details of this device see pages 302 to 306 Tschappat’s O & G. =Powder Bags.= Cartridge bags for separate loading are made of raw silk, and are sewed with silk thread. Other materials are apt to produce flare-backs or premature explosions because they are not entirely consumed in the bore or continue to burn if not consumed. The raw silk however either is entirely consumed or if not, the parts ignited immediately go out as soon as the flame is removed and do not smoulder. Specially treated cotton fibre bags have been tried but so far as I know have not as yet superseded the raw silk for the purpose. The gases remaining in the bore after the discharge of a charge of smokeless powder are explosive and with air form an explosive mixture, hence the danger upon opening the breech if any smouldering particles remain in the bore. [Illustration: _21 GRAIN PERCUSSION PRIMER MARK II-A_]