7. Put in the latch, the latch-spring; put on the latch-cover and screws. 8. Insert the breech-block with carrier-ring into the gun. 9. Put in the hinge-pin and screws. 10. Lock the block. Experiments are now being made with the view of introducing metallic ammunition, and it is possible that a breech mechanism similar to the Driggs-Schroeder or the Gerdom will be adopted. A general description of each is therefore given, and also the proposed method of supplying ammunition. THE DRIGGS-SCHROEDER BREECH MECHANISM. [Illustration: FIG. 37_a_. The cuts show the positions of the block when locked, when unlocked, and when revolved to the rear. ] The breech-block (_b_), weighing 31 lbs., has bands (_c_) on top and sides that fit in corresponding grooves in the jacket; these bands have an inclination of 2½ degrees to the front and upward to secure complete obturation and facilitate locking and unlocking. There is a transverse hole (_i_) through the block for the main bolt (_d_); it is lengthened in a nearly vertical direction so that the first motion of the operating-handle causes, by the action of the main cam (_e_), the block to descend and disengage the bands from the grooves in the breech, after which the block rotates to the rear and rests on the tray. Guide-bolts (_k_), screwed through each side of the curtain, project into guide-grooves (_l_) cut in each side of the block and confine its movement. The main cam, firing-pin and spring, and sear and sear-spring are contained in the block, the front face of which bears a heavy removable plate (_a_). The firing-pin has under its rear part full-cock and half-cock studs; and the sear, actuated by its spring, presses up against it and engages the studs in succession as the firing-pin is forced to the rear by the action of the main cam against the lug on the under side of the firing-pin. The point of this lug always rests in the circular groove in the upper rear face of the main cam. The extractors (_j_), one on either side, lie flat against the rear face of the tube of the gun and in recesses in the front face of the block. They revolve on pivots which work in recesses in the curtain. The rear sides of the tails of the extractors and the lower front corners of the breech-block form cam surfaces so arranged that when the block _rotates_ to the rear the extractors extract and eject the cartridge-case. The spring lock on the handle prevents the main bolt and block from moving, when the breech is closed, under stress other than that applied directly to the handle. TO DISMOUNT. 1. Back out the guide-bolts far enough to clear the guide-grooves. 2. Half-cock. 3. Take off handle. 4. Tap the main bolt gently to start it and withdraw it completely, supporting the block with the hand. 5. Lower the block out of place, tilting it slightly to the rear so as to clear the extractors. THE BREECH-BLOCK BEING OUT, TO TAKE THE MECHANISM APART.