SMG: U.S. M3A1 “Grease Gun”

US SMG M3A1 Grease Gun Tilt

SMG M3A1 Manufacturer Guide Lamp, Ithaca
Cartridge 11.43×23mm Overall Length 29.8″
Action Blowback Barrel Length 8″
Magazine 30rnds dtch box Weight 7.9 lbs


The M3A1 actually arrived a little late for broad issue in WWII, but it improved and polished on a legendary SMG.  Let’s briefly cover some history and just what was updated to make this American classic.

Entering WWII, the Thompson M1928A1 was the standard submachine gun of the United States.  Remember, submachine gun technology emerged in WWI and progressed somewhat slowly until the late 1930s in most nations.  So, while reliable, the Thompson was very much overbuilt and required far too much machine time and too many important materials to produce.  It was heavy, large, and cumbersome.

In 1941 the Ordance Department began searching for an alternative.  They were pleasantly surprised by the brutally simple British Sten gun, which functioned with a simple open-bolt, blowback, welded metal design.  But, the changes were just too many and it was hard to overcome a prejudice against such a crude looking weapon.  A wood stocked and more conventional design, still easier to manufacture than the Thompson, provided by George Hyde was selected as the U.S. SMG M2.  Production, however, floundered as material demands and manufacture needs for even this simplified SMG doomed its adoption.

Returning to the drawing board, the Ordnance Department remembered the Sten and came up with the following requirements for the new U.S. submachine gun:

  • All metal construction
  • Must make extensive use of stamped metal components
  • Should require a minimum of machine operations
  • Should use few or no critical metals (i.e. superior steel)
  • Must be fully automatic with low cyclic rate

George Hyde, of the failed M2, was determined to arm the U.S. and returned with an entirely new gun.  His improved prototype met all the demands of the Ordance Board and in tests functioned reliably, accurately, and under extreme abuse.  It could be produced for a fraction of the cost of the Thompson and in less time with simpler manufacturing.  It was compact, light, easy to maintain, and straight-forward to use.  The only real problem with Hyde’s gun was that everyone found it so damned ugly.  But looks don’t matter much in war and so this new gun was adopted as the U.S. Submachine Gun Caliber .45, M3 in December of 1942.

US SMG M3A1 Grease Gun Comparison with Thompson M1A1

Scale comparison of Thompson M1A1 and M3A1 “Grease Gun”

Now, we’ll cover this revolutionary design in better detail someday.  For now, let’s focus on the fact that once issued there were some additional mechanical problems to be addressed.  This isn’t a surprise as production began only two months after submission!  The biggest hang up was that the M3 used a cocking handle on the right side of the action, not unlike a jack in the box.  This proved to be a weak point in the design, prone to failure and racking up additional machine work and assembly time.  It was dropped and with it a series of improvements undertaken and an improved model M3A1 was adopted in December of 1944.  It included the following:

  • Handle replaced with simple finger hole in bolt body
  • Ejection port lengthened to allow soldier to pull bolt all the way back with finger through port
  • Cover spring strengthened
  • Barrel grooves added to ease disassembly
  • Magazine release fitted with flared guard to prevent accidental release
  • Wire stock reshaped to provide dual use as a barrel wrench
  • Wire stock given shaped protrusion to aid with loading magazines
  • Larger oiler placed in pistol grip

US SMG M3A1 Grease Gun Barrel jazzThe changes to simplify the cocking assembly also made the new M3A1 easier to disassemble overall, so now the barrel could just be unscrewed right off the receiver.  With basic maintenance tools built into the gun, servicing was a breeze in the field and the guns were better kept as a consequence.  Roughly 15,500 were produced by Guide Lamp before the end of the war halted manufacture. While the M3A1 became the new standard, the M3 continued to serve alongside.  At first, M3s sent for repair were only given an upgraded housing and magazine release shield but during the Korean War many were upgraded with replacement parts to the M3A1 configuration.  These parts, along with 33,000 fresh M3A1 SMGs were provided by Ithaca.

So, the U.S. SMG M3A1 ends up being a simple blowback operated, open bolt gun.  It is made entirely of metal, mostly stamped and wire steel of “inferior” quality compared to previous firearms.  It feeds from a 30-round, double stacked, detachable box magazine.  To operated the bolt a soldier opens the dust cover and inserts his finger into a milled hole in the bolt body, retracting it to the rear where it locks open.  If he pulled the trigger the bolt would be released to spring forward on two symmetrical springs with guide rods.  It would pick up a cartridge, chamber it, and fire it all in one stroke.  The force of the discharging cartridge drives the bolt rearward until it locks behind the trigger sear again.  A rudimentary safety is provided by a protrusion of metal under the dust cover.  When closed, it prevents the bolt from traveling forward and therefore prevents fire.US SMG M3A1 Grease Gun right

Both the M3 and M3A1 were popularly issued to paratroops and tank crews due to their extremely compact nature.  When initially issued soldiers of every branch were dumbfounded.  The guns looked nothing like anything they had seen before and their crude and ugly appearance must have made them seem downright dangerous.  They were mocked and given a variety of derisive nicknames, the most popular of which was “grease gun” because they looked so much like the mechanic’s tool.  Skepticism quickly faded, however, once these SMGs were put to the test.  Troops found they had all the power of the Thompson in a compact, extremely lightened, and reliable package.  In the end that mocking name has become one of pride and is still recognized today as an American classic.

While the M3 saw service in WWII, the M3A1 only saw limited issue before the end of hostilities.  It was issued in greater numbers during the Korean War, Vietnam War, and sometimes beyond.  Ultimately, these submachine guns parallel the humble Jeep in that they didn’t look like much but grew to become a dependable friend to the U.S. soldier.


Special thanks to our friends at the South Carolina Military Museum for letting us borrow this piece.


2 Responses to “SMG: U.S. M3A1 “Grease Gun””

  1. Joey Wood says:

    Great article! Never realised the “grease gun” was such a solid weapon, like the troops I just looked at it & assumed it had to be awful & a stop-gap!

  2. Carl joseph says:

    As a member of the 2nd Tank Bn, 2nd Marine Div. in 1973 we had grease guns in the armory. Unfortunately I never got the opportunity to fire one.

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