What Caliber Is A Thompson Machine Gun

The .45 ACP Round and the Gun That Made It Famous

Ask anyone what they picture when you say "Tommy Gun," and chances are they'll describe a drum-fed submachine gun slicing through the 1920s. But here's the thing — when people ask what caliber is a Thompson machine gun*, they're usually after something more specific than just the nickname.

The short answer: the classic Thompson fires .Day to day, 45 ACP. But that's only the beginning of the story.

What Is a Thompson Machine Gun?

The Thompson isn't really a machine gun in the technical sense. It's a submachine gun — selective fire, meaning it can switch between semi-automatic (one shot per trigger pull) and fully automatic (holding the trigger fires continuously). Still, invented by General John T. Thompson in 1918, it was originally designed to give infantry a portable weapon that fired a powerful pistol cartridge.

The .45 ACP (Automatic Colt Pistol) cartridge was chosen deliberately. Worth adding: at the time, the U. S. military was transitioning from .In real terms, 30 caliber rifles to the larger . 45, believing the heavier bullet would stop threats more reliably at close range. The Thompson brought that same logic to a compact, shoulder-fired weapon.

The .45 ACP Cartridge

The .45 ACP round is a rimless, tapered cartridge that typically fires a 230-grain bullet at around 850 feet per second from a pistol barrel. Plus, in the Thompson's longer barrel, velocity increases slightly. The round is known for its stopping power and relatively low recoil compared to rifle cartridges — a key reason it became popular in submachine guns.

Why It Matters

The Thompson's choice of .Think about it: before the Thompson, most automatic weapons were heavy rifle-caliber machines that required tripods or vehicle mounts. It influenced how law enforcement and military units thought about close-quarters combat. In real terms, 45 ACP shaped more than just its ballistic performance. The Thompson was portable enough to carry through city streets or urban combat zones.

That portability changed tactics. On top of that, gangsters in the 1920s and 1930s knew they were facing something different when federal agents or police pulled out a Thompson. The weapon's distinctive sound — a rapid, throaty bark — became as intimidating as its actual firepower.

The .45 ACP chambering also meant the Thompson could share ammunition with the M1911 pistol, simplifying logistics for units carrying both weapons.

How the .45 ACP Powers the Thompson

Blowback Operation

The Thompson operates on simple blowback principle. When the firing pin strikes the primer, the powder charge pushes the bullet down the barrel while simultaneously forcing the heavy bolt rearward. Even so, the bolt's mass and the recoil spring's tension control how quickly the action opens. By the time the bolt cycles back, pressure has dropped enough that it's safe to extract the spent casing and chamber a new round.

This system works well with the .Consider this: 45 ACP because the cartridge generates relatively low chamber pressures compared to rifle rounds. The blowback design is simple, reliable, and easy to manufacture — important qualities when you're producing weapons at wartime scale.

Magazine Options

The Thompson's most iconic feature is its 50-round drum magazine, though it also accepts 20- and 30-round box magazines. The drum feeds rounds in a spiral, allowing high capacity in a compact package. In practice, though, the drum was prone to feeding issues if not kept clean and properly loaded.

The .In practice, 45 ACP's tapered case design helps with reliable feeding from both drum and box magazines. The round's shape naturally guides it into the chamber, reducing jams compared to straight-walled pistol cartridges.

Barrel and Sights

The Thompson's barrel length varies by model. Now, military versions typically had barrels around 10 inches, while civilian semi-automatic variants often sport shorter barrels to comply with National Firearms Act requirements. The iron sights are simple notch-and-post, calibrated for close-range engagement where submachine guns excel.

Common Mistakes and Misconceptions

Confusing Caliber with Cartridge

Lots of people mix up "caliber" and "cartridge.Think about it: " The . 45 ACP refers to a specific cartridge, not just the bullet diameter. While the bullet is indeed .451 inches in diameter, the entire cartridge — case dimensions, powder charge, primer type — defines what the Thompson fires. Practically speaking, you can't just stuff any . 45-caliber bullet into a Thompson and expect it to work.

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Assuming All Thompsons Are the Same

The Auto-Ordnance company produced several variants of the Thompson over the years. Some were made for military contracts, others for law enforcement, and later models for civilian sales. While they all chamber .45 ACP, details like barrel length, stock configuration, and receiver materials vary significantly.

Overlooking the Semi-Automatic Versions

Modern civilian-legal Thompsons are semi-automatic only. They look identical to the full-auto originals but fire one shot per trigger pull. In practice, the . 45 ACP chambering remains the same, but the shooting experience is completely different.

Practical Tips for Understanding Thompson Caliber

Know Your Ammunition Types

The .45 ACP comes in several loadings suitable for the Thompson. Standard ball ammunition (full metal jacket) is what military and law enforcement traditionally used. Consider this: jacketed hollow points offer better expansion for self-defense scenarios. +P loads provide higher velocity but increase wear on the weapon.

Maintenance Matters

The .And regular cleaning of the chamber, bolt face, and feed ramp is essential for reliable function. Consider this: 45 ACP generates significant carbon buildup, especially when using the high-capacity drum magazines. The Thompson's design makes field stripping straightforward, but the drum magazine requires careful attention to keep its spiral feeder clean.

Legal Considerations

In many jurisdictions, owning a fully automatic Thompson requires special licensing, tax stamps, and background checks. Because of that, semi-automatic civilian versions are more accessible but still subject to various state and federal regulations. Always check local laws before handling or purchasing any Thompson variant.

FAQ

What other calibers were used in Thompsons?

Some limited production runs experimented with 9mm Parabellum and .38 Super, but these are extremely rare. So the vast majority of Thompsons were built for . 45 ACP.

Can you use modern .45 ACP ammo in a vintage Thompson?

Yes, but it's wise to use standard-pressure loads rather than high-velocity or +P ammunition, which can stress older components.

What's the difference between a Tommy Gun and a Thompson?

Nothing. "Tommy Gun" is just the popular nickname for the Thompson submachine gun.

Are .45 ACP Thompsons still made today?

Auto-Ordnance still produces semi-automatic versions for civilian sale, chambered in .45 ACP.

Why was .45 ACP chosen over 9mm?

The U.S. Worth adding: military's preference for the larger . 45 round during the early 20th century drove the choice. The bullet's diameter and weight were believed to offer superior stopping power.

The Last Word

The Thompson's .In practice, 45 ACP chambering wasn't just a technical decision — it was a statement. Which means when the weapon first appeared, it represented a shift toward portable automatic firepower that could be wielded by individuals rather than crews. That .45 ACP round, developed for the M1911 pistol, found its perfect partner in a weapon that would become iconic.

Today, whether you're looking at a museum piece, a registered fully automatic example, or a civilian semi-automatic replica, the .45 ACP remains central to what makes a Thompson a Thompson. The round's distinctive report, its proven stopping power, and its historical significance all trace back to that simple question: what caliber is a Thompson machine gun?

The answer shaped a century of firearms development.