Initially, the American military was somewhat slow in procuring Thompsons, since it was felt that the new M1 carbine would supersede the submachine gun. After Pearl Harbor, it was obvious that both were needed and ever increasing quantities of Thompsons were ordered. By February 1942, some 500,000 M1928A1s had been produced.

The Thompson had acquired the nickname “Tommy Gun” early on and the name stuck. Auto-Ordnance recognized the value of the nickname and soon patented it. Thompsons were referred to as Tommy Guns both in and out of military service, and the term became almost generic for all submachine guns.

The Thompson was the only submachine gun in mass production in any of the allied nations during the early days of World War II. Great Britain made good use of the many thousands of Thompsons acquired under Lend-Lease, and they were a favorite of the elite Commandos (who preferred them by far to the Sten, and kept their Thompsons well after D-Day) as well as other British units. These Lend-Lease M1928A1 Thompsons were essentially identical to the versions produced for the U.S. except that the horizontal fore-end was replaced by the earlier vertical model and the buttstock sling swivel was mounted on top of the stock rather than on the bottom in U.S. fashion.

US Marine Corps Photo

Very early production M1928A1s had the English spelling “Calibre” on the receiver, although some of these were procured by the U.S. Fairly large numbers of these Lend-Lease Thompsons have been reimported back into the United States since the end of World War II. They can be further identified by the typical British proofs on the receiver and barrel. Eventually the Thompson would be supplanted by other submachine guns in British service, especially the Sten, but the Thompson remained in use by our allies throughout the war.

With demand increasing, Auto-Ordnance searched for ways to simplify the Thompson and increase its production rate. The most obvious candidate for change was the finely made adjustable Lyman rear sight. It was expensive and required much machining time. Realistically speaking, such a sight was superfluous and overly complicated for a submachine gun. In December 1941, a simple stamped, non-adjustable, sheet metal peep sight was approved for the Thompson. This speeded production and reduced the cost, but did not materially affect the gun’s usefulness.

Another feature that received some attention was the barrel cooling fins. These were finely finished on the commercial Thompsons, but as production continued, the fins were squared off and eventually eliminated. Again, this had little or no negative impact on the gun’s performance. The Thompson had a thick barrel in relation to its cartridge, and the fins were of marginal value in cooling.

Another short-cut was eliminating the checkering on the safety and fire-selector levers, and they were replaced with simplified stick-type levers. Stick-type levers were not as easy to manipulate, and the trade-off between manufacturing time and utility could be debated.

On later guns, the finely checkered actuator knob was replaced by a smooth one. This also had no impact on performance. Another change in the 1928A1 was the addition of a metal reinforcing bolt that strengthened the buttstock.

These changes did not occur at once, but were phased in over a period of time. M1928A1s may be observed with all the production simplifications, only one or two, or with none at all. The M1928A1 Thompson is divided into three basic variants:

(1) finned barrel, adjustable rear sight

(2) finned barrel, fixed rear sight

(3) smoother barrel, fixed rear sight

Serial numbers aside, the general time frame in which a M1928A1 was manufactured can be deduced by it configuration in relation to the above major production changes. That is, the fewer the changes from the original design, the earlier the gun.

The M1928A1 was made in greater numbers than any other variant, with some 562,511 guns made between 1940 and early 1943. Of these, almost 300,000 M1928A1 Thompsons made during 1941 and 1942 were supplied to our allies, chiefly Great Britain. It has been reported that as many as 200,000 may have been lost enroute to England on ships sunk by the Germans.

Even with these modifications, the basic mechanism remained unchanged. To make a significant impact upon reducing manufacturing time and cost, all aspects of the gun had to be re-evaluated. The Savage engineering team proposed a radical redesign of the Thompson that centered around the elimination of the Blish locking device. The Blish lock was always of dubious value and required a great deal of machining time. Most contemporary submachine guns used a simple straight blowback system made possible by the use of pistol cartridges. Savage assembled an experimental blowback Thompson and it worked quite well. Despite some initial resistance by Auto-Ordnance, the merits of the design were too great to overlook and a prototype was fabricated and tested by the Ordnance Dept. in 1942. The redesigned Thompson functioned every bit as well as the M1928A1 and could be manufactured much faster and cheaper. It was adopted in April 1942 as the “Submachine Gun, Caliber .45 M1.”

The M1 differed from the M1928A1 in the following ways: the M1 was a straight blowback-operated arm that eliminated the Blish locking system; the cocking handle (actuator) was moved from the top to the right side of the receiver; the Cutts Compensator was eliminated; the M1928A1’s removable buttstock was replaced by a fixed stock; and the M1 accepted only box magazines.

The M1 was simplified even more by the substitution of a fixed firing pin on the bolt face rather than the separate hammer used with the M1928A1 and the M1. This change resulted in the adoption of the “Submachine Gun, Caliber .45 M1A1” in October, 1942, although it did not see full-scale production until early 1943. Most M1A1s incorporated metal guard “ears” on either side of the stamped rear sight to protect it.

Total M1A1 production was 539,142 guns by the time it ceased in 1944. In addition, a number of M1 models were converted to M1A1 configuration by ordnance depots and arsenals.

The fact that the simplified Thompsons could not be used with the drum magazine did not have a serious impact on effectiveness, though the drum was popular with some users. The following passage regarding the drum magazine  appears in the Rosenquist and Buerlein book Our Kind of War. “On an amphibious assault, Raiders liked to land with a 50-round drum locked into the weapon. When that was emptied, it was simply discarded and immediately replaced by a 20-round box magazine.”

The 50-round drum was criticized by many as being too awkward, heavy and prone to rattle—all obviously undesirable traits in combat—and the drum was dropped from widespread use fairly early in the war.

To compensate for the loss of the drum’s firepower, a 30-round magazine (essentially a lengthened version of the 20-round magazine) was adopted, and it could be used with all models of Thompsons in service including the M1928A1. Some soldiers taped two or three magazines together in an attempt to make reloading faster, but the real value of this can be debated.