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McGlashan Air Machine Gun


Internal Working Parts of the Receiver - Navy Model

Other Pictures:
The Aerial Gunnery Trainer E-13

E-13 Gun Mechanism

Navy Model N

Loading, Here done with a coke bottle

US Navy training with the Model N

The MacGlashan Air Machine Gun
Because no single definitive and comprehensive history of the MacGlashan machine guns has been found, and probably does not in fact exist, what follows has been compiled from a number of manuals, books, reports and magazine articles, most of which are listed in the Bibliography. Appreciation is hereby expressed to their respective authors.

The first known appearance of the MacGlashan Company was in 1937, when it started manufacture of a BB air rifle.

Coming,history of the MacGlashan family, the MacGlashan Air Gun Company, and of the development of the guns.]

Various models of MacGlashan compressed air machine gun were produced. The first were made for use by shooting galleries at amusement arcades. With the advent of World War II, three models were produced for the U.S. armed forces for use in aerial gunnery training. They were:

- N (USN)
- E-3 (USAAF)
- E-13 (USAAF)

Electricity was supplied from regular 110-volt ac power through a power pack unit that converted it to 24 volts DC . Compressed air, in a pressure range of 60 to 200 psi, was provided by a separate air compressor that normally supplied all the guns at a range installation via a manifold. BBs were conventional commercial .177 caliber copper-coated steel shot.
When the trigger was pulled (actually, a switch was closed) power flowed to an electrical solenoid that moved a couple of levers to perform several functions. First and foremost, it opened a valve that admitted compressed air to a piston. This pushed the piston to the rear, which withdrew a plug and allowed compressed air to enter the breech and fire a BB. Second, it actuated a rotary ratchet driven disc that was similar in function to the cylinder of a revolver. It had ten holes in it, each of which conveyed a BB from a tubular magazine to a point immediately above the breech, at which point it was pushed downwards into a short vertical feed tube by a spring-loaded ball detent that also held the disc in position. The vertical feed tube held four BBs, and whenever a BB was fired, a replacement dropped into the breech. A spring-loaded detent projected into the barrel just far enough to retain the BB until sufficient air pressure built up behind it to propel it down the barrel.

The piston also pushed a spring-loaded recoil block to the rear. This block, as it neared the limit of its backward travel, opened a breaker switch in the firing circuit which de-energized the solenoid, allowing it to return to its original position. This closed the air valve, allowing the spring-loaded piston to move forward and plug the rear of the breech. The recoil block also, by virtue of its mass and inertia, provided timing for the firing cycle, since it took a finite time interval for it to move back and then forward again. As it moved forward it closed the firing circuit, enabling the entire cycle to start again. The shaking produced by the longitudinal motion of the various moving parts probably helps the flow of BBs from the magazine into
the feed mechanism.

Timing is critical in the proper operation of the gun. Specifically, it is essential that power (electric and compressed air) remain on until the BB has left the barrel and the recoil cycle is well under way. Also, it is important that power not be restored until the moving parts are almost back in battery (forward position). This is controlled by the relationship between the recoil block and the breaker switch. The details of the breaker switch actuation varied from model to model

The cylindrical body of the primary valve has a reservoir capacity of about one cubic inch of compressed air, assuring that an ample volume is immediately available for operating the gun, not limited by the small-diameter supply line.

Ballistics Simulation
Since one purpose of the BB machine gun was to familiarize the gunner with the characteristics of the actual machine guns he would be firing, some thought was given to at least partially simulating the ballistic performance of the .30 caliber machine gun. It turns out that the angular gravity drop of a BB at 1,000 inches closely approximates that of a cal. .30 M1 bullet at a range of 556 yards if the BB`s muzzle velocity is just over 600 feet per second. The muzzle velocity can be controlled by adjusting the air pressure.

Since 556 yards is 20 times 1,000 inches, a scale factor of 1/20 was chosen. Since it is physically impossible to simulate simultaneously both the gravity drop and the required lead angle, the former was chosen. The Navy astutely solved this dilemma by displacing the front sight laterally so that the sight line was offset from the bore axis. Thus the student gunner visually applied the full long-range .30 caliber lead using his sights, while actually applying only the smaller lead to the gun that was required to score hits with the short-range BBs. Since the targets moved at a constant speed in the same direction and at the same range, they could get away with this solution.

MacGlashan provided the following ballistics data:

Muzzle velocity: 609 feet per second
Time of flight to 1000 inches: 0.21 second
Range, inches 500
750
1000
Drop, inches 0.4 1.1 2.2

With gun fixed in vise, an average of 74% of shots fired will group within 8½ inch circle on 1000 inch range.

The physics of gravity drop would seem to require a muzzle velocity substantially higher than 609 fps in order to produce the stated drop distances. That said, the data are presented for whatever they may be worth.

The Navy Model - N
The Navy model, Type N, officially known as Device 3-A-8, Air Machine Gun, may have been the same as MacGlashan`s commercial model. It can be identified primarily by the slightly-tapered perforated barrel jacket that covers the rear two thirds of the barrel. This was purely cosmetic, since a compressed air gun is self-cooling by virtue of the expanding air during firing.

Being a hand-aimed weapon, this gun was characterized by spade grips and a push-button trigger at the rear of the receiver. It also had a horizontal tubular BB magazine mounted above the receiver. This magazine actually consisted of two concentric tubes, with the BBs stored in the annular space between the inner and outer tubes. This configuration permitted the installation of an E-5 telescopic sight inside the inner tube, so the gunner was actually sighting through the magazine.

The trigger switch and the interrupter switch were combined into a single assembly that was characterized by open contacts at the end of leaf springs, similar to relay contacts. The interrupter contacts were pushed into contact with each other by a rod projecting downwards from the recoil block.

The primary air valve closure consisted of a cylindrical part roughly resembling a golf tee (with a blunt end), restrained with a single cantilevered wire. This configuration insured that the valve would be kept closed by the air pressure behind it were its spring to break.

The Navy purchased 424 of these guns for just over $110,000, or about $259 per set. How the unit cost was allocated between the gun, the mount and the power supply is not known.

The Aerial Gunnery Trainer Type E-3
The AAF apparently initiated procurement of its E-3 Aerial Gunnery Trainer in 1941, since it was declared a standard AAF item on July 28, 1941. It was a redesigned version of MacGlashan`s commercial model, modified to eliminate some unsatisfactory characteristics (probably having to do with timing and consistency of rate of fire) . The first aerial gunnery school was being set up at Las Vegas that summer, and the initial purchase was very likely part of the equipping of that school, and of the others soon to follow..

The E-3 resembled the Navy model externally, except that the perforated barrel jacket was absent, and a tubular extension was added to the muzzle.

In this gun, the primary valve exit is closed by a simple steel ball held in place by a single loop cantilevered wire spring. If this spring were to break, the ball would be free to roll around inside the valve body, permitting a continuous flow of air to escape.

The E-3 served throughout the war, and was declared obsolete on November 6, 1945.

The Aerial Gunnery Trainer E-13
The E-13 was a variation of the E-3 intended for use in fixed gunnery training for fighter pilots. It was mounted at the front of a stripped-down Link trainer-like cockpit. The student “flew” the trainer to aim and fire at a target that moved along a track in front of the trainer. A reflector sight was mounted above the instrument panel, and aligned with the gun.

Since there was no need for sights mounted on the gun itself, a single-tube magazine was oriented vertically, which probably increased ammunition utilization and facilitated loading. Also, this model had no spade grips or trigger on the back, since the trigger switch was mounted on the pilot`s stick grip.

There were internal changes as well, mostly having to do with electrical components and the means of actuating them. On the assumption that later models of the gun contained improvements designed to correct shortcomings encountered in earlier models, it appears that there may have been problems with electrical contact life in the earlier design, perhaps due to arcing and pitting and perhaps breakage due to metal fatigue. At any rate, the E-13 used an enclosed acro switch with a capacitor connected across the contacts to reduce arcing. Acro snap switches were designed to have a snap action and to provide a wiping motion between the contacts as they opened and closed, helping to keep them clean and smooth. They were also designed for long life under demanding conditions.

A more complicated latching mechanism was added to the underside of the recoil block to operate the acro switch at the correct points in the firing cycle. It was presumably more reliable and less subject to breakage and maladjustment than its predecessor.

Also, in the E-13 primary valve, the steel ball is held seated by a coil compression spring contained in a housing that also served to restrain the travels of the ball itself. This spring probably proved more reliable than its predecessor. The Xcelite buffer was moved to the front end of the recoil block, and an adjustable recoil spring was added to cushion the rearward travel of the recoil block, and presumably to enable fine-tuning of the cyclic rate.
The E-13 was declared standard on June 1, 1943 and obsolete on November 6, 1945. A recorded serial number on a specimen examined suggests that at least 444 of these guns were made.

Weapontype:
BB machine gun

Manufacturer:
McGlashan Air Machine Gun Company

Operation:
blowback using compressed air

Chambers:
.177 in (4.5 mm)

Magazine Capacity:
900-1000

Feed system:
ratchet

In Production:
1940 - 1944

Sights:
Iron or optical (1 X)

Rate of fire:
up to 600 rpm

Effective range:
80 feet (25 m)

Muzzle Velocity:
up to 600 ft/s (180 m/s), depending on air pressure

Extern links:
http://macglashanbbgun.com/introduction/

 

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