The M39 submachine gun.

The M39 is an improved version of the Heckler and Koch UMP submachine gun that serves as the US military’s primary submachine gun. It largely replaced the M41 submachine gun because of its lighter weight and smaller size, although it provides the user with slightly less firepower.

Weapon Designation:Submachine Gun, 9mm, M39
Type:Submachine Gun
Place of OriginGermany
In Service:2048-present
Production History:
Manufacturer:Heckler & Koch GmbH
Weight:4.4 lbs (2 kg)
Length:28 in (713 mm) (stock open)
18.62 in (473 mm) (stock closed)
Barrel Length:8 in (200 mm)
Cartridge:9x19mm Parabellum
Action:Straight blowback
Rate of Fire:600 rounds/min cyclic
Muzzle Velocity:1,550 ft/s (472 m/s) with M990D fluted round
Muzzle Energy:480 ft/lbs (651 Joules) with M990D fluted round
Effective Range:200 m (219 yd)
Feed System:30-round detachable box magazine
Sights:Optics rail (SO-4 reflex sight as standard)
Iron sights


Despite some limitations, the M41 submachine gun was regarded as an excellent weapon overall and the passage of the Cooperative Exploration Treaty and the included Future Weapons Development Ban in 2036 led to plans for the M41’s improved variant, the M41A1, to remain the US military’s primary submachine gun for the foreseeable future.

However, by the mid-2040s, around fifteen years after the M41’s adoption, some further disadvantages to its design had become apparent. The primary advantage of the M41 was its explosive ammunition, but despite this many troops found it larger and heavier than they liked, and the recoil was also cited as being somewhat high for a weapon of its class.

The P90 was briefly considered as a supplement to the M41 because of its small size and light weight, but ultimately a weapon firing explosive ammunition was more desirable. However, the desire for both lower weight and less recoil meant that a different cartridge than the 10x24mm round would need to be used, and the military was reluctant to accept a new cartridge for logistical purposes. Eventually, Armat Battlefield Systems proposed an explosive variant of the 9x19mm Parabellum round. The 9x19mm round was already in use by the US military as the primary cartridge for the M58 and M4 pistols, which meant that the explosive round could also be used for those pistols as well and entirely replace existing types of 9mm ammunition so that an entirely separate cartridge would not have to be introduced into supply chains. Unfortunately for Armat, their cartridge proposal was accepted but their weapon design lost out to a slightly modified version of Heckler and Koch’s UMP, which was adopted as the M39 submachine gun.

The M39 largely replaced the M41 submachine gun except in certain specialist roles. Compared to the other two most similar weapons in the military’s inventory, the M41 and the M933, the M39 is smaller and lighter than both weapons, has a smaller magazine than the M41 but equal capacity to the M933, lower accurate range than the M933 but more firepower within that shortened range, and less range and firepower than the M41 but also less recoil.


The M39 is very similar to the UMP submachine gun, with only a few differences. It uses more advanced polymers which further decrease the weapon’s weight, features a muzzle brake that drastically reduces recoil at the cost of an extra 23mm of overall length, and is provided exclusively with a slim-line handguard with no tactical rails. However, there is a ten-slot tactical rail on the top of the receiver for mounting various optics, including the SO-4 reflex/remote sight.

When firing high-pressure loadouts of 9mm like the M990D and the M9116E, the M39 has a maximum effective range of 200 meters, which is less than the M41, but the M39 is also a full two pounds lighter and even with its side-folding stock in the open position is only 18mm longer than the M41A with its collapsing stock in the closed position. Additionally, because of both the lighter cartridge and the muzzle brake, the M39 has virtually no recoil. The smaller diameter and shorter length of the 9mm round also mean that less explosive can be carried, making the M9116E cartridge less powerful than the 10x24mm explosive caseless rounds, but still superior to the 6.2x45mm round used by the M933P carbine within the M9116E’s effective accurate range of 200 meters.

Because of the ammunition it fires, the M39 uses a traditional mechanical cartridge ignition system rather than the electronic system used by the M41. It does, however, feature an ammunition counter on its right side like the M41. Despite carrying the designation “M39” in the US military, all M39s have “UMP” engraved on the side above the bolt release.

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