remedied by further modifying the action through the addition of greater weight to the
O iT> n piston. The first rearward movement of the bolt was thereby somewhat retarded.
Swebilius deserves great credit for accomplishing this most difficult task, especially since it was performed in a few weeks' time. In this short period he made the Marl in gun a reliable automatic arm that was used throughout the war and for 3 years afterwards as the principal synchronized automatic machine gun of the American air force. Later it was also adapted to tank use. The weapon met with considerable enthusiasm on the western front as the following cablegram from A. E. F. headquarters in February 1918 shows: 14 Marl in aircraft guns have been fired sue-cessfully on four trips from 18,000 to 15,000 feet altitudes at a temperature of -20 degrees Faliren heit. On one trip guns completely covered with ice. Both metallic links and fabric belts proved satisfactory."
Development, of the M.irlin aircraft, gun was primarily one of modification and refinement from the gas swinging lever Colt '95 model. Mar I in was so successful in the undertaking that new features were constantly being added. On 1 January 1918, the Signal Corps requested the design of a different firing mechanism that would permit single or automatic shots when used with four-bladcd propellers and with the new and improved Nelson mechanical synchronizing gear. An arrangement whereby the hydraulic and mechanical trigger motors could be attached to the front of the lock container was also desired. The lock container was to be redesigned and the hammer materially lightened to increase the rate of fire.
An informal test was held on 27 December 1917, at which time another type of hydraulic synchronizing gear, also manufactured by the Marl in Rockwell Corp. and similar to the Con-stantinesco gear, was tried out at rates varying from 200 to 600 rounds per minute with a total angle of dispersion of 03°. It was the closest and most accurate synchronization accomplished in this country with any type of machine gun so far. The Marl in aircraft machine gun has the distinction of being the first gas-operated weapon to be synchronized successfully.
The arm employed either a fabric or a disintegrating metal link bell that could be made up with as many as 500 rounds. Actually metallic links were generally used in aircraft throughout the war because of the inconvenience of disposing of the fabric belt's loose end after firing.
On 8 January 1918, a conference was held by the Army Ordnance Department at New Haven to decide on changes to be incorporated in the firing mechanism. Both the Signal Corps and the Marlin-Rockwell Corp. submitted new designs. Tests in the last days of that month proved the
Marlin-Rockwell device satisfac tory and the Sig-
nal Corps design a total failure because of easy-breakage of parts. This was accountable for both by inferior material and by generally poor construction. A new hydraulic trigger motor was adopted at the same time and a contract for 15,000 motors and modified firing mechanisms was placed with Marlin-Rockwell. By this time the alteration and redesign of so many parts meant that they were not interchangeable with similar components of the old guns. To distinguish it in nomenclature from its original parts,
Marlin Aircraft Machine Gun, Model 1917, Cal. .30, the improved product was called the Marl in Aircraft Machine Gun, Model 1918.
The modified weapon's principal point of difference from the 1917 model lay basically in its
ability to be adapted to fire single shot or full automatic. This permitted much closer synchronization than full automatic alone. At the conventional propeller speeds of the day, however, the rate of fire was almost as "teat as with the o automatic principle.
One of its main points of diffcrcncc from the components of the 1917 model was a reduced gas pressure in order to cut down the parts breakages and stoppages caused by recoil, it was accomplished by enlarging the size of t he gas piston and by drilling out less of the gas adjuster screw. The latter increased the volume of expansion of the gas chamber in the cylinder and cushioned its action. Hy milling three slots about one six
/ o teenth of an inch in diameter through the side of the gas adjuster barrel, a vent was provided for the gas in the cylinder. It could be opened or closed at will by means ol the adjuster. The double purpose was thus achieved of satisfactorily cushioning the recoil and also affording a
wide range of adjustment, of gas pressure acting on the piston. The increased recoil power necessitated a change in the bolt's cam slot pin to eliminate excessive breakage. Mote stock was added to the rear end of this piece to strengthen the cross section at a point where fracture was most frequent.
The fin was also given a glass-hard treatment followed by spot annealing. Material of the bolt pin was changed to chrome nickel steel to increase its durability as breakage often resulted
from the severe wear on it after the first few hun dred rounds. The newly designed and more
/ o powerful hammer action also proved too strenuous for the firing pin assembly and the pin had to be redesigned with a long gradual taper throughout its entire length. This made it not only stronger but at the same time more flexible.
In order to eliminate accidental firing by con
tact of the firing pin with the extractor, a three sixteenth inch piece of stock was added at the top of the latter's lug employed to support the front of the bolt, it was also found necessary to j bevel oil the end of the receiver on the left hand side directly behind the ammunition belt. Interference at this point with the base of the cartridge had a tendency to tvvisL t he belt and cause
O j stoppage. A hole was drilled through the right side of the lock container in which a key. known as the functioning operating cam, was inserted. Rotation of this key raised the trigger, thus enabling the gun to fire full automatic for testing and firing without the synchronizing gear.
An interesting device added to the new Marlin was known as the jam preventer. It consisted of a small steel stamping which was applied without further alteration to the ratchet lever in place of the ratchet lever pin washer. Its function was to prevent the lever's pawl from engaging Lhe feed wheel when the cartridge was in the act of feeding and thus blocking the stop against further rotation of the wheel. The device would have been more appropriately named an anti-double-lceding device. According to reports from the field a reduction of stoppages resulted from use of the jam preventer. Unloading of the gun could also be accomplished by this arrangement without removing the belt as was heretofore found to be necessary.
Marlin Aircraft Machine Gun, Model 1918, Cal. .30
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