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United Defense UD-42 (Model 42 / M42)

Submachine Gun (SMG) [ 1942 ]

The United Defense UD-42 submachine gun failed to succeed the storied M1 Thompson line but found service during World War 2 with various partisan groups.

Authored By: Staff Writer | Last Edited: 02/26/2018 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site.

A myriad of varied weapon types proliferated the market during, and after, World War 2. Many were pushed into existence by the sheer necessities of war, brought about by materials rationing, basic supply-and-demand shortages, and economical/logistical measures. The United States had committed to World War 2 at the end of 1941 following the Japanese attack on Pearl Harbor and its warplanners (and local war industry) were now thrust into yet another global conflict that it was ill-prepared for.

This led to many new products being put forth by various hopeful entrepreneurs and companies attempting to secure that lucrative serial production contract. The United Defense Model 42 ("M42") was a compact submachine gun initiative designed by Carl Swebilius as a possible replacement for the storied M1 Thompson submachine gun - an expensive, complicated, and maintenance-heavy design from the previous World War. The new weapon - in its prototype form - was chambered for the same .45 ACP cartridge as the Thompson and featured a wooden shoulder stock, machined metal body, and forward vertical grip. The trigger unit was underslung in the usual way with the pistol grip formed as part of the shoulder stock. The action was of a simple blowback operation and, to provide the operator with a quick-reloading feature, the 20-round magazine boxes were welded together along their length as one collective unit - providing up to 40 rounds "at-the-ready". Sighting was through a fixed front post and a rear adjustable assembly.

Overall weight was 10lbs and overall length was 32 inches with an 11 inch barrel. A select-fire feature allowed for controlled ammunition use while rate-of-fire reached 700 rounds per minute with a muzzle velocity listed at 1,100 feet per second. As a short-to-medium-range man-stopper, the M42 could prove an effective wartime implement.

The end result seemed to fit the bill - the M42 looked the part and could very well succeed the costlier M1 Thompson line by its simpler approach. Its finalized form was rechambered for the ubiquitous 9x19mm Parabellum German pistol cartridge - readily available all over the world - and allowed the operator to utilize any captured stocks of enemy ammunition as a result.©MilitaryFactory.com
In practice, the gun was anything but successful for American forces and not adopted for standardization going forward. The design was found to be no more easier to produce on a large scale than the Thompson for it still required much machining to bring it into existence. In-the-field, the weapon was noted for its propensity to collect dust, dirt, and the various other common battlefield debris - in turn requiring a rigid maintenance schedule to keep the weapon in working order. There also proved issues with the distinct magazine arrangement which had a tendency to warp over periods of extended use, jamming the weapon at the feed mechanism during the worst possible moments. As such, the M1 Thompson lived to see further days in American service and the M42 was passed along to specialist forces during the remainder of the war. Some 15,000 examples were ultimately manufactured by United Defense Supply Corporation from the period of 1942 to 1943.

The gun ended in the hands of resistance elements across China, Crete, Czechoslovakia, France, Italy, the Netherlands, and the Philippines where it found use as a serviceable weapon under combat conditions. The Office of Strategic Services (OSS) - forerunner to the modern American Central Intelligence Agency (CIA) - was also a high-level recipient of this weapon series and used to for its varied clandestine operations throughout the remainder of the war. Stocks remained in service even after the conflict for many foreign operators - and managed extended service lives into the Cold War years that followed.©MilitaryFactory.com
Note: The above text is EXCLUSIVE to the site www.MilitaryFactory.com. It is the product of many hours of research and work made possible with the help of contributors, veterans, insiders, and topic specialists. If you happen upon this text anywhere else on the internet or in print, please let us know at MilitaryFactory AT gmail DOT com so that we may take appropriate action against the offender / offending site and continue to protect this original work.


Service Year

United States national flag graphic
United States


Submachine Gun (SMG)

United Defense Supply Corporation - USA
(View other Arms-Related Manufacturers)
National flag of China National flag of Czechia National flag of France National flag of Italy National flag of the Netherlands National flag of the Philippines National flag of Taiwan National flag of the United States China; Czechoslovakia; France; Italy; Netherlands; Philippines; Taiwan; United States
(OPERATORS list includes past, present, and future operators when applicable)
Special Forces
Qualities of this weapon have shown its value to Special Forces elements requiring a versatile, reliable solution for the rigors of special assignments.

Overall Length
820 mm
32.28 in
Barrel Length
279 mm
10.98 in
Empty Wgt
9.04 lb
4.10 kg

Adjustable Rear; Front Post


Blowback; Select-Fire

Blowback Operation
Gas pressure from the rearward movement of the ignited cartridge case provides the needed bolt movement, ejecting the spent case and stripping a fresh case from the magazine.
(Material presented above is for historical and entertainment value and should not be construed as usable for hardware restoration, maintenance, or general operation - always consult official manufacturer sources for such information)


9x19mm Parabellum

Sample Visuals**

Graphical image of a 9mm pistol cartridge
Rounds / Feed

20+20 Round Detachable Box Magazine
Cartridge relative size chart
*May not represent an exhuastive list; calibers are model-specific dependent, always consult official manufacturer sources.
**Graphics not to actual size; not all cartridges may be represented visually; graphics intended for general reference only.
Muzzle Velocity
1,100 ft/sec
(335 m/sec)

UD42 "Marlin" - Base Series Designation
Model 42 ("M42") - Alternative Designation

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