is provided for use in ordnance shops. This wench should be clamped tightly around the breech end of the barrel with the handle extending to the right, The barrel can then be started by a quick downward movement of the handle. The receiver may be held in a vise or by means of a block of wood inserted up between the side walls of the receiver. b. The components attached to the barrel should never be removed except when replacements are necessary or for purposes of salvaging. Unscrew the flash hider with the combination tool. Drive out the front-sight key pin, drive the front-sight base off to the front, and remove the front-sight key. The gas cylinder tube bracket can then be driven off to the front after the pin has been driven out. These parts should be replaced in the reverse order. c. When replacing a barrel always be sure that it is tight enough in the receiver never to work loose. Screw the barrel into the receiver until the draw line matches that of the receiver. Then assemble the gas cylinder tube to see if the gas cylinder tube bracket on the barrel lines up properly. If it does not, the barrel should be turned very slightly until the alignment is correct. 33. Butt stock and buffer group.—a. The butt stock bolt can be unscrewed by inserting a long screw driver through the hole in the butt plate after the butt plate trap has been opened. To do this with the combination tool the -SLIDE BUFFER -BUFFER PRICTION CUP BUFFER FRICTION CONE BUFFER SPRING -BUTT STOCK BOLT SHOWING BUFFER MECHANISM Plate 28. butt plate must first be removed by unscrewing the butt plate screws, long and short. As soon as the butt stock bolt has been loosened the butt stock can be withdrawn to the rear. To strip the butt plate, unscrew and remove the butt plate trap spring screw and spring. Then drive out the butt plate trap axis pin and remove the butt plate trap. b. The removal of the butt stock allows the stripping of the buffer mechanism. Unscrew the buffer nut and remove to the rear in the order mentioned, the buffer spring, the four sets of buffer friction cups and cones, and the buffer. The buffer tube is threaded into the receiver, and should never be removed except for replacement. In order to assemble the butt stock and buffer mechanism, reverse the method given above. 34. Rear-sight group.-The rear sight is to be removed or stripped only when a replacement of certain part or parts is necessary. Remove the rearsight base spring screw and drive the rear-sight base out to the rear Unscrew the rear-sight leaf axis screw and nut and take out the rear-sight leaf. The rear-sight base spring can then be removed from the base. Unscrew the rear-sight slide stop screw, which will allow the slide to be withdrawn from the leaf. With a small drift drive out the rear-sight slide catch axis pin and take off the rear-sight slide catch and spring. To reassemble the above components reverse the order given for stripping. 35. Receiver group.-a. To strip the operating handles press in on the operating handle plunger and push out the operating handle plunger pin toward the countersunk side of the plunger. The plunger and spring can then be removed. Reverse the above method in reassembling. b. The bolt guide spring may be lifted out of its seat in the bolt guide with the rim of a cartridge. The guide and spring can then be removed from the receiver. To reassemble these parts, insert the longer turned-over end of the bolt guide spring in its hole on the inside of the receiver. Then replace the bolt guide and hold it while the spring is pushed over until the shorter turned-over end engages the groove in the bolt guide. The change lever stop and spring can be removed after the change lever stop spring pin has been driven out. This should never be done except when necessary. 36. Gas adjustment.-a. The rifle should normally be operated on the smallest port, and this setting will never be varied unless the rifle shows signs of insufficient gas. To align the smallest port, screw in the gas cylinder with the combination tool until the shoulder of the gas cylinder is about one turn from the corresponding shoulder of the gas cylinder tube and the smallest circle on the cylinder head is toward the barrel. Lock the cylinder in position. If, upon firing, the rifle shows signs of insufficient gas, try setting the cylinder one complete turn on each side of the original setting. As soon as the proper setting has been obtained the rifleman will carefully note the position so that he can quickly assemble the cylinder to the proper point without trial. The larger ports are provided for use in case of emergency, when the action of the rifle has been made sluggish through the collection of dirt and grit, or the lack of oil, and the conditions render it impossible for the riflemen to correct these troubles. For this reason the threads should be kept clean and oiled and the cylinder free to turn if necessary. The extractor, ejector, and the chamber of the barrel should be examined and cleaned and defects corrected when possible. Under adverse conditions, and when signs of insufficient gas become apparent, the cylinder should be unscrewed onethird of a turn, thus registering the medium circle and aligning the medium port with the gas orifice. Repeat this operation in order to connect the largest port with the barrel. Excessive friction or dirt may sometimes result in the failure of the action to complete the forward movement under the action of the recoil spring, which part may also have become permanently set or short from continued use under conditions of excessive heat. In such case replace the recoil spring. b. Results of insufficient gas.-(1) Failure to recoil (usually due to a misaligned or excessive clogged gas port, or extremely dirty mechanism). (2) Failure to eject. (3) Weak ejection. (4) Uncontrolled automatic fire (exceptional). c. Results of too much gas.--(1) Excessive speed, causing pounding. (2) Excessive heat in gas operating mechanism. 37. Notes for repairmen.—The following notes are prepared for the use of repairmen: Filing, or otherwise altering parts, should only be done under the strictest supervision, as this often fails to remedy the trouble and results in damage to the rifle. Filing and altering of parts are sometimes necessary, but should never be done except by an expert repairman, under the direction of a commissioned officer competent to supervise the work. All defective parts should be distinctly marked or tagged to avoid their future use. a. Barrel.—(1) The barrel should not be removed until worn to such an extent as to require replacement. This wear usually occurs in the rifling near the breech or in the chamber. A barrel is usually good for 8,000 rounds, after which time the accuracy depreciates quite rapidly. A worn or pitted chamber may result in ruptured shells, or in the shells swelling and sticking in the chamber, rendering extraction difficult or uncertain. (2) The gas port through the barrel and gas cylinder tube bracket may become stopped up and not allow sufficient gas to pass through to insure positive action. To remedy this difficulty clear the port with the gas cylinder cleaning tool. The lower surface, especially that portion immediately surrounding the port of the gas cylinder tube bracket, should fit closely to the corresponding surface of the gas cylinder tube. Some wear at this point can be taken up by bending the lips of the bracket down slightly, tapping them lightly with a hammer, or by bending the retaining rib of the tube back at each edge, thus bringing the surfaces into closer contact. A smooth, close fit is desired. If the fit is excessively tight, some metal may be removed from the surface of the gas cylinder tube bracket by the use of the special hand scraper provided for this purpose, or with an oilstone or dead smooth file. Care should be exercised not to remove too much metal, especially from that surface near the port. b. Gas-operating mechanism.-(1) The front retaining rib of the gas cylinder tube may become too loose in the gas cylinder tube bracket, resulting in insufficient gas and requiring adjustment, as described above, or replacement. The wall of the tube may become dented, thus binding the piston. In some cases it may be possible to straighten this out until the piston works freely. In replacing a gas cylinder tube see that it correctly fits the bracket and that the rear hole lines up properly in the receiver. The tube should never be twisted to secure a fit at this point. In case of necessity a reamer may be run through the rear hole to clean out a small amount of metal. (2) The ports of the gas cylinder may become clogged to such an extent as to render the action uncertain, or burnt powder deposited on the inside of the gas cylinder, causing the piston to stick. The gas cylinder cleaning tool may be used to clean these parts. Failure to operate, due to insufficient gas, is sometimes the result of wrong assembling of the gas cylinder, screwing it in one turn from the shoulder, Unscrew one-third of a turn to change to the medium port, and one-third of a turn farther to the large port. The relative sizes of the ports are indicated by circles on the front end of the cylinder. The threads of the gas cylinder should fit snugly enough to prevent excessive gas leakage. The fit of the gas cylinder and piston, although free, should not be excessively loose, in order that the recoil may be uniform. Extreme wear would necessitate replacement. In replacing a gas cylinder make sure that the piston works freely when the cylinder is set at each port. Test as described below. (3) If the gas piston has become bent it may bind in the gas cylinder or tube. Slight bends may be straightened and tested in the following manner: Remove the trigger guard, bolt, and recoil spring. The piston and slide should work back and forth easily as the muzzle of the rifle is tilted first up and then down. Particular attention should be given at the point where the piston enters the cylinder. In replacing a piston, the new one should be tested in this manner. In the case of the old-type piston, which is threaded rigidly into the slide, slight bending may be necessary in order to secure the proper alignment c. Receiver.-Wear in the receiver usually occurs on the locking surface or on the bolt supports. The rear cam surface of the bolt supports must be accurately positioned in order to cam the bolt lock upward at the proper time. The bolt supports must be tightly in the receiver and be well riveted. The hammer used in riveting should be heavy enough to swell the rivets throughout their entire length. An excessively loaded cartridge may spread the side of the receiver slightly and loosen the bolt supports. In this case the sides of the receiver should be carefully brought back into position, with the aid of a vise, and the bolt supports reriveted. d. Firing mechanism.—(1) The slide should fit freely in the receiver and the sides should be straight. The slide should not be excessively loose in the receiver, as this may cause the hammer pin to batter the top of the front end of the slot in the left side of the receiver, rendering it difficult to pull back the operating handle when cocking the rifle. To remedy this difficulty the slot can be smoothed over and widened slightly by filing. Slight cracks may occur in the slide, extending from the hammer pin hole to the upper surface. This does not in general affect the operation of the rifle. (2) The hammer should have a good bearing against the seat in the rear of the slide, and when assembled should not be loose enough to allow any appreciable tilting when pressure is exerted in the upper portion of the front face. In addition to this condition, excessive wear or deformation of the front face may be the cause of misfires. When the parts are in their forward position the hammer should clear the firing pin, but not by more than one thirty-second of an inch. The hammer should come up against the link when the gas cylinder tube is not in place. The recoil spring should not become set, and hence too short for positive operation. In some cases this can be corrected temporarily by pulling out the spring. In some cases replacements will be necessary. The recoil spring should be 15 inches long. (3) In addition to the actual length of the firing pin the following points determine the distance which the firing pin will protrude through the bolt in the locked position (causing pierced primers at one extreme and misfires at the other): The distance from the face of the hammer to its rear bearing surface against the slide, the distance from this surface to the front stop shoulder of the slide, the distance from the slide-stop shoulder of the gas cylinder tube to the gas cylinder tube retaining pin hole, the distance from this hole to the face of the receiver, the distance from the face of the receiver to the locking surface and the distance from the locking surface of the bolt lock to the front face of the bolt. Firing pin protrusion can only be gauged when the rifle is assembled and the bolt is in the locked position. It will be necessary to use a gauge extending from the muzzle of the gun, or to take a wax impression of the projection when the bolt is locked. (4) When the bolt is unlocked the bolt link should bear on the locking shoulder of the bolt lock and should hold the bolt lock up so that it just clears the bolt supports and guide as the bolt is moved rearward. Frequent blown or pierced primers may cause the rear head of the firing pin to batter the front of the bolt link. This battered surface should not be allowed to develop a sharp edge which might catch or break off the head of the firing pin. (5) The top rear corner of the firing pin head should be well rounded to avoid the above-mentioned breakage. The shank of the firing pin should be straight and the point not excessively deformed or pitted. The retracting cam should be beveled on each side, and neither this part nor the corresponding slot in the bolt lock should show battering or signs of misalignment. (6) The bolt and bolt lock are an assembled unit and should never be separated. There should be a good bearing between the two so that none of the shock of the recoil is taken on the bolt lock pin. The bolt lock pin may become loose with wear until one end projects and catches on the inside of the receiver, resulting in stoppages and mutilation of the receiver. Test by pushing the pin both ways with the finger. It should not be possible to cause it to project on either side. In case of projection, rerivet the pin. Considerable wear on the locking shoulder of the bolt lock, in the joint between the bolt and bolt lock, or on the face of the bolt, may result in excessive head space, causing ruptured cartridges and, in some cases, blown primers. During the rearward motion the bolt lock should never bear to any appreciable extent on the bolt supports or guide. If the firing pin hole in the face of the bolt is enlarged or off center, the assembly should be replaced, as this is often the cause of breakage of firing pin points. (7) When the gun is assembled without the recoil spring, but with the gas cylinder and gas cylinder tube in place, the recoiling parts should slide back and forth freely as the gun is turned muzzle up and vice versa, unless the breeching is snug. In that case they should move freely when the bolt lock is disengaged from the recoil shoulder of the receiver. On the forward movement the bolt lock should be cammed up by the rounded rear ends of the bolt supports, but there should be sufficient clearance between the bolt supports and the locking shoulder of the receiver to prevent excessive binding. When in the locked position the front shoulder of the bolt should just clear the rear shoulder of the barrel. If the bolt comes up against the barrel but does not interfere with the locking of the bolt lock, the operation of the gun will not be affected unless dirt is allowed to accumulate in front of the bolt. (8) The extractor should have particular attention to see that it has retained its proper shape and that the cartridge fits the hook correctly. The inner radius of the extractor hook should closely fit the bottom of the cannelure of the cartridge. The extractor notch, just back of the hook, should |