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The Tommy Gun: History of the M1A1 Thompson Submachine Gun

The Tommy Gun: History of the M1A1 Thompson Submachine Gun



The Thompson submachine gun, also known as the Tommy gun, is a .45-caliber, air-cooled, blowback-operated weapon. An American icon, U.S. infantrymen carried the Thompson in the European, North African, and Pacific theaters of World War II and during the Korean War. Firing .45 ACP cartridges at an approximate rate of 600–700 rounds per minute, the Thompson proved to be a reliable and effective weapon on the battlefield. 

Origins of the Thompson 

A submachine gun is a carbine designed to fire handgun ammunition and is capable of fully automatic or burst fire. It allows the soldier to deliver devastating firepower at close range.

The Thompson submachine gun design was spearheaded by U.S. Army brigadier general John T. Thompson in cooperation with three design engineers: Theodore H. Eickhoff, Oscar V. Payne, and George Goll. John T. Thompson sought financing and established the Auto-Ordnance Company to develop and manufacture the weapon. As the firm completed the first prototypes after World War I ended, it had to seek new markets for its weapon. 

The Thompson saw widespread use in the 1920s and ‘30s. It was popular among police and sheriff’s departments, detective agencies, banks, mining companies, armored car services, and criminals. 

Gangster firing a Tommy gun

Gangster firing a Tommy gun

Both Prohibition-era organized crime syndicates and Depression-era bank robbers saw the Thompson’s potential, hence the nickname “The Chicago Typewriter.” The United States Marine Corps also bought the weapon to deploy in Central and South America during the Banana Wars.

Winston Churchill inspecting a Tommy gun

Winston Churchill inspecting a Tommy gun

In 1938, the U.S. military adopted the Thompson SMG. There are three military variants of the Thompson: The M1928A1 (the interwar model), the M1, and the M1A1. 

Design Details 

The M1 and M1A1 Thompson are simplified variants designed to reduce the cost of production. The M1928A1, and its predecessors, used the Blish principle, a type of friction-delayed blowback. In this system, a bronze lock is forced against inclined locking grooves machined into the steel receiver. 

M1928 Thompson without the Cutts Compensator

M1928 Thompson without the Cutts compensator

Under pressure, these metals adhere to each other, creating a mechanical disadvantage and delaying the action from opening until the pressure in the chamber drops to a safe level. While not suitable for full-power rifle cartridges, it was compatible with the relatively low-pressure .45 ACP cartridge.

As the pressure in the chamber drops, the pressure on the lock subsides, allowing it to move up, disengaging the locking grooves in the receiver. The purpose of delayed-blowback systems is to reduce the weight of the bolt necessary to keep the breech closed during firing without compromising safety.

To simplify the action, the M1 substituted simple blowback operation, which relies on the static inertia of the bolt and the tension of the recoil spring to keep the breech closed. This change proved effective.

M1 Thompson (note the side charging handle)

M1 Thompson (note the side charging handle)

Firing Modes

There’s a selector lever on the left side of the frame; behind the magazine catch, you can rotate to choose between semi-automatic and fully automatic fire. At the rear of the frame is a two-position safety catch. When the selector is set to semi-automatic, pressing the trigger once will fire one round. If you want to fire another round, you need to release the trigger, allow it to reset, then press it again. 

Tommy Gun Fire Selector Switch

Tommy gun fire selector switch and safety switch

When the selector is set to fully automatic, pressing the trigger will fire the weapon continuously until either the trigger is released or the ammunition in the magazine is depleted. 

Firing the Thompson 

Retracting the bolt cocks it on the sear, which locks it open. Pressing the trigger causes it to lower the sear, releasing the bolt and allowing the recoil spring to drive it forward. The bolt strips a round from the magazine on the forward stroke, feeds it into the chamber and fires it. Firing from an open bolt allows air to circulate through the barrel, cooling the weapon after extended firing sessions.

When the cartridge is chambered, the extractor snaps over the rim and into the extracting groove. As the propellant charge ignites, it generates expanding gases. These gases exert pressure against the breech face through the cartridge case head, overcoming the bolt’s inertia and forcing it to the rear.

During the rearward stroke of the bolt, the extractor withdraws the cartridge case from the chamber. The case head strikes the ejector, which is fixed in the receiver, causing it to exit the weapon through the ejection port. 

The bolt compresses the recoil spring. If the selector is set to semi-automatic, the sear will catch the bolt. If the selector is set to fully automatic and the trigger is depressed, the sear will remain in the lowered position. The recoil spring expands, driving the bolt forward and repeating the cycle.

Magazine Types

The Thompson submachine gun is a magazine-fed weapon. Before the M1 and M1A1 variants, the Thompson accepted 50- and 100-round detachable drum magazines. While the 50-round drum increased the firepower available to the soldier, it was also heavy, bulky, and noisy. Later models would feed from 20-round stick magazines, which proved more reliable under adverse conditions.

Other Changes

On the M1928A1, the charging handle is located on the top of the receiver and reciprocates in a slot. In the M1 and M1A1, the charging handle is located on the right side. 

The M1928A1 had a Cutts compensator designed to reduce muzzle climb during fully automatic fire by exhausting propellant gases upward. As this was deemed unnecessary, it was eliminated on later models. Likewise, the radial cooling fins on the barrel were removed, saving an additional machining operation. The adjustable rear leaf sight was also replaced with a less complicated fixed “L” sight.

The M1 retained the separate firing pin and triangular hammer of the M1928A1. In this design, when the bottom lobe of the hammer strikes the front of the receiver, it rotates, driving the firing pin into the primer and firing the cartridge. The M1A1 substituted a fixed firing pin machined into the face of the bolt, eliminating four parts (the firing pin, firing pin spring, hammer, and hammer pivot pin). 


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Every SMG Needs a Companion

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