The Thompson submachine gun, an icon of 20th-century warfare and Prohibition-era gangster lore, is most famously chambered for the .45 ACP (Automatic Colt Pistol) cartridge. But this specific caliber choice defined the weapon’s identity, giving it the heavy hitting power and subsonic ballistics that made it a formidable close-quarters battle tool. So while the . 45 ACP remains the standard answer, the Thompson’s history includes a few notable variations and experimental calibers that are essential for a complete understanding of this legendary firearm.
This is where a lot of people lose the thread.
The Standard: .45 ACP (11.43×23mm)
When John T. Thompson and his Auto-Ordnance Corporation finalized the design that became the Model 1921, they built it around the .45 ACP round. This was not an arbitrary decision. The U.S. Still, military had adopted the . 45 ACP in 1911 alongside the M1911 pistol, largely based on combat experience in the Philippines where the .38 Long Colt proved insufficient at stopping determined attackers That's the part that actually makes a difference..
Counterintuitive, but true Simple, but easy to overlook..
The .45 ACP cartridge fires a heavy, large-diameter bullet (typically 230 grains) at relatively low velocities (approximately 850–950 feet per second from a pistol barrel, slightly higher from the Thompson’s longer barrel). This combination delivers massive kinetic energy transfer and creates a large permanent wound cavity without over-penetration—a critical factor for military police and trench clearing operations.
For the Thompson, the .Which means 45 ACP offered distinct tactical advantages:
- Subsonic Performance: The standard load remains subsonic, allowing for effective use with sound suppressors (though suppressors were rarely issued with Thompsons in WWII, the potential existed). On top of that, * Magazine Compatibility: The cartridge’s rimless, straight-walled case fed reliably from both the iconic 50-round and 100-round drum magazines and the later 20- and 30-round box magazines. * Controllability: Despite the Thompson’s high cyclic rate (600–1,200 rounds per minute depending on the model), the .45 ACP’s moderate recoil impulse allowed trained operators to maintain reasonable burst control.
Military Variants and the M1/M1A1 Simplification
The most produced versions of the Thompson—the M1 and M1A1—retained the .Now, 45 ACP chambering but simplified the internal mechanics to reduce manufacturing costs and complexity. The early Model 1921/1928 utilized a complex Blish lock delayed-blowback system. The M1/M1A1 switched to a simple straight blowback operation with a heavier bolt and stronger recoil spring.
This change did not alter the caliber, but it did slightly affect the cyclic rate. That said, regardless of the model designation—whether it was the "Chicago Typewriter" in the hands of law enforcement or the "Tommy Gun" carried by GIs in Normandy—the ammunition remained the standard . Plus, the M1/M1A1 typically cycled around 600–700 rounds per minute, slower than the 1921’s 800–1,000+ RPM, making the later models slightly more controllable during sustained fire. 45 ACP ball (M1911) or later M1917 tracer/armor-piercing variants That's the part that actually makes a difference. Practical, not theoretical..
The Forgotten Calibers: .38 Super and 9mm Parabellum
While the .45 ACP defines the Thompson in popular culture, Auto-Ordnance experimented with other calibers to secure international contracts.
The Model 1923 and .38 Super
In the late 1920s, Auto-Ordnance developed the Model 1923, a heavy-barrel variant intended as a light machine gun or "trench broom" replacement. Some prototypes were chambered in .38 Super (9×23mm SR). The .38 Super offered higher velocity and a flatter trajectory than the .45 ACP, with better penetration against early body armor and vehicle bodies. That said, the U.S. military rejected the concept of a submachine gun in a pistol caliber other than .45, and the Model 1923 never entered mass production That's the part that actually makes a difference. Simple as that..
The 9×19mm Parabellum Prototypes
Seeking sales in Europe and South America, where the 9mm Parabellum (9×19mm) was the standard pistol cartridge, Auto-Ordnance produced a small number of Thompsons chambered in 9mm.
- The Model 1926/1928: A handful of guns were converted or built for 9mm Parabellum for trials in countries like France, Sweden, and China.
- The Belgian Connection: The most significant 9mm production run was manufactured under license by Fabrique Nationale (FN) in Belgium. Designated the MP 1934 (or sometimes Model 1934), these guns featured a different bolt face, extractor, and magazine housing to accept 9mm magazines (often MP 18 or Suomi-style stick magazines, though some used modified Thompson drums).
- Chinese Copies: During the Second Sino-Japanese War, Chinese arsenals (such as the 21st Arsenal in Nanjing and later the 51st Arsenal) produced Thompson copies. While most were .45 ACP, some late-war or post-war productions experimented with 9mm due to the prevalence of captured Japanese and imported German ammunition.
These 9mm variants are exceedingly rare today and represent a fascinating "what if" in the Thompson lineage. They generally featured a higher cyclic rate due to the lighter 9mm bolt and higher pressure cartridge, often exceeding 900 RPM It's one of those things that adds up..
The .22 LR Trainer: The Model 1927A5
In the modern era, the Thompson legacy continues through semi-automatic only replicas manufactured by Auto-Ordnance (now a division of Kahr Arms). To make easier affordable training and recreational shooting, they produce the Model 1927A5 (and earlier 1927A3/A1 variants) chambered in .22 Long Rifle (.22 LR).
These are not select-fire machine guns; they are closed-bolt, semi-automatic carbines with 16.5-inch barrels to comply with U.While they share the external aesthetics of the WWII M1A1—including the horizontal foregrip and blued finish—the internal mechanics are entirely different, utilizing a simple blowback action designed for the rimfire cartridge. S. Here's the thing — national Firearms Act (NFA) regulations regarding barrel length and overall length. This allows enthusiasts to experience the ergonomics and handling of the "Tommy Gun" at a fraction of the ammunition cost That's the part that actually makes a difference..
Ammunition Types Used in the .45 ACP Thompson
Understanding the caliber also requires understanding the specific ammunition issued for the platform during its service life Small thing, real impact..
| Ammunition Designation | Bullet Type | Purpose / Notes |
|---|---|---|
| Cartridge, Ball, M1911 | 230 gr FMJ (Full Metal Jacket) | Standard issue combat load. Round nose profile for reliable feeding. Practically speaking, |
| Cartridge, Tracer, M1917 | 172 gr Tracer | Red tip. Used for signaling and trajectory observation (every 5th round in belts/drums). |
| Cartridge, Armor-Piercing, M1918 | 163 gr AP (Steel Core) | Black tip. Designed to penetrate light armor/vehicles. Rarely issued for SMG use. |
| Cartridge, Blank, M1921 / M1 | Paper/Wax bullet | Training only. Required a blank-firing attachment (BFA) to cycle the action. |
Dummy, M1921 | Inert (No powder) – Intended solely for safety demonstrations and training scenarios requiring a functional-looking projectile without risk of injury.
Beyond these primary calibers, the Thompson platform has inspired a modest number of contemporary replicas that attempt to capture both the aesthetic and mechanical essence of the original weapon. On the flip side, while no full-scale production clone of the classic M1A1 exists in large numbers, several smaller manufacturers have released variants using the same basic rotating drum mechanism and fixed-chamber design. These typically feature updated finishes—such as matte black, metallic silver, or even camouflaged colors—and improved sight systems while retaining the iconic Tommy gun silhouette. Some have incorporated modern safety features and trigger mechanisms that reduce the inherent recoil of the short-recoil system, making them suitable for training without sacrificing the characteristic kick and balance of their ancestors.
The significance of the Thompson extends beyond historical military utility into the realm of cultural iconography. Its distinctive sound—an unmistakable, high-pitched crack that could echo across battlefields—has become synonymous with aggressive intimidation and rapid fire capability. In popular media, from classic films like The Thief of Bagdad to video games and television series, the automatic rifle has served as a dramatic shorthand for overwhelming force. Even in civilian contexts, the principle of a portable, rapid-fire weapon continues to influence modern security equipment, such as the compact automatic rifles developed during the Cold War and various personal defense tools sold today under the guise of lawful self-defense.
From a technical standpoint, the evolution of the Thompson’s ammunition reflects broader trends in small arms development. 22 LR variant, in particular, benefits from the lower recoil impulse of the rimfire cartridge, allowing shooters to practice precise target acquisition without the physical demands associated with full-power pistol ammunition. 45 ACP rounds to the lightweight .22 LR trainers illustrates how engineers can maintain a platform’s core identity while adapting it to new operational requirements. The transition from heavy-duty .Because of that, the . This makes it an ideal bridge between dedicated military training and casual recreational shooting, democratizing access to the unique tactile experience of operating a Tommy gun.
Also worth noting, the story of the Thompson is one of adaptation and resilience. Despite being retired from active service after World War II, the weapon never disappeared entirely. Here's the thing — its design principles influenced subsequent automatic weapons, including the M60 and even certain early versions of the AK-47’s intermediate-caliber concept. The legacy of John Browning himself endures in every generation of replica and derivative, ensuring that the spirit of the “buzzsaw” lives on in both history books and hobbyist workshops alike Simple, but easy to overlook. That's the whole idea..
All in all, whether examining the rare 9mm variants of the pre-war Thompson or the accessible .As technology advances and new materials emerge, the Thomson design will likely find fresh adaptations, but its fundamental appeal remains unchanged: a weapon that combines audible authority with mechanical simplicity, capable of delivering a decisive strike at any range. 22 LR Trainers produced by Kahr Arms, the thread connecting all iterations is a commitment to the iconic, powerful image of the automatic rifle. Plus, each iteration honors a distinct chapter in firearm history—from the grim efficiency of mid-century warfare to the controlled precision of modern training ranges. The Thompson stands as both a relic of a bygone era and a living testament to the enduring ingenuity of American engineering Still holds up..
