Difference between M1921 and M1928 explained.
The M-1921 was the first Thompson submachine gun commercially produced. Exactly 15,000 were made during 1921-22 by Colt under contract from Auto-Ordnance Corporation (which had no production capability). Their serial numbers ranged from 41 to 15040. These were all built for speculation or inventory and were warehoused by Auto-Ordnance. Sales trickled out but were not robust. There were no military contracts at this time.
The firing rate of the M1921 is 800 to 900 rounds per minute.
In 1928 the U.S. Marines were having a little "police action" in the jungles of Nicaragua. Wanting a better close combat automatic weapon than the BAR, they bought a few M1921 Thompsons for trial. They liked it but thought it was wasteful of ammunition. They agreed to buy a quantity provided the firing rate could be reduced to about 600 rounds per minute.
Auto-Ordnance modified existing M1921 guns to slow their firing rate. Doing this they added mass to the actuator (riveted on a bar of steel), and replaced the recoil spring, its guide rod and buffer. The Marines liked it and initially ordered 500, adopting it into the military as the "Model of 1928."
Auto-Ordnance filled this order from existing inventory of M1921 guns, modifying each as mentioned. On each gun so modified they hand stamped an "8" over the "1" of the "Model of 1921." rollmark. Above this modified rollmark they hand stamped "U.S. Navy". They also offered the same modification commercially, marked the same way. Collectors refer to these as the 1928 Navy or the 1921-28 overstamp. All of these came from original inventory of 15,000 guns and bear their original serial numbers. These all have the Cutts Compensator, whereas some of the original 1921 guns were sold without a Cutts (another whole story in itself).
This is a photo from a Julia auction:
The modification required absolutely no change to the receiver or lower frame. Thus, owners of a M1921 can switch to M1928 firing configuration simply by switching the original Colt internal parts mentioned with army surplus M1928A1 parts (actuator, recoil spring, guide rod, and buffer). The gun then shoots exactly as a M1928. For someone as clumsy as me it takes about 2 minutes to switch back and forth.
The original inventory of 15,000 Colt made guns was still slow to sell during peacetime between WW1 and WW2. However, as WW2 started brewing in Europe in the late 1930s, interest developed in this weapon. With the opening of hostilities, orders soon exhausted Auto-Ordnance's inventory. Still having no production capability, they sought new production by a contractor.
Colt refused to make any more Thompsons, saying the "gangster era" had nearly ruined their reputation. To this date Colt has never made another Thompson; thus, the original 15,000 Colt guns are the darlings of collectors.
Savage took up the contract and eventually made close to 1.5 million Thompsons. These began with the serial number 15041. They were made to M1928 specs and initially were marked "Model of 1928". On adoption by the U.S. Army they became the "Model of 1928A1". Later in the war Auto-Ordnance developed manufacturing capacity and produced several hundred thousand of these.
The Model of 1928A1 went through several design changes, becoming the Model M1, followed by the M1A1.
The
only M1921 Thompsons ever made were the original batch of 15,000 made in 1921-22.
That's the short story. For full details there are quite a few reference works out there.
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