After the war, Thompson became very interested in, if not obsessed with, the concept of a “trench broom,” as he called it, for close quarter fighting. He recognized that the .45 ACP cartridge would be ideal for use in such an arm. The heavy slug could provide a great deal of short range stopping power, particularly when fired from a full-automatic arm.
While working on a semi-automatic rifle design, Thompson became enamored of a locking mechanism developed by a U.S. Navy officer, John B. Blish. The Blish principle used a sliding wedge as a locking device. Thompson chose to adapt the Blish locking mechanism to his “trench broom.”
Thompson assembled a talented team of designers and, by 1919, the new firm of Auto-Ordnance was involved in developing the first American submachine gun to see production. Several magazine designs were evaluated, but it was decided that box magazines and a novel drum magazine would offer the best service.
The most natural market for Thompson’s new gun was, of course, the U.S. Military. When he brought up the general concept in 1917 to a number of Ordance Dept. officers they expressed interest. The Navy also thought that such an arm might be of value to Marine and Navy landing party duties.
By early 1920, Auto-Ordnance was ready to have its prototype tested by the government. On April 27, 1920, the Springfield Armory conducted preliminary functioning tests and the results were very impressive, as 2,000 rounds were fired with only one stoppage.
The Marine Corps tested the gun a few months later at Quantico, Virginia, with similarly favorable results. Even though no official trials had been scheduled, Auto-Ordnance began plans to have the new gun mass produced. Contracts were eventually negotiated with Colt for the manufacture of 15,000 “basic firing mechanisms” and with several other firms including Lyman and Remington, for items like sights and walnut stocks. They were to be assembled by Auto-Ordnance at its own facility.
The new arm was designated “Thompson Submachine Gun, Model of 1921.” It was finished in Colt’s attractive bluing and fired at a rate of approximately 800 rounds per minute. Both the Army and Marines tested the M1921 Thompson, and although the results of the tests were favorable, adoption of the submachine gun was not recommended. The military was not particularly interested in acquiring any new weaponry in the early 1920s. Auto-Ordnance made a number of inquiries to foreign government during this period, but few orders were forthcoming. The company had a little better luck with the commercial market, though sales were slow.
The Thompson submachine gun eventually gained a reputation as a gangster weapon due to its widely publicized use in the hands of notorious criminals of the era, but he weapon was not as ubiquitous as the public may have been led to believe. Sales to law enforcement agencies increased as many municipalities, as well as the FBI, felt obliged to obtain Thompsons so as not to be outgunned by the crooks. Frankly speaking, the need for submachine guns by many law enforcement agencies during this period may have been more perceived than real.
Although it was not officially adopted, the Marines obtained several hundred M1921 Thompsons for use in Nicaragua, where they proved quite valuable. A number were also issued to the Marines guarding the mail during a rash of armed robberies. The Navy eventually decided that the Thompson would be useful in the arms racks of some of its vessels, particularly the Yangtze River gunboats.
In 1928, the Navy made plans to adopt the Thompson but requested modifications from the M1921 design. They mandated a lower 600-round cyclic rate, a horizontal fore-end and a Cutts compensator. The Cutts had been an option on Thompsons, and it helped hold down the muzzle during firing. With these changes, the “U.S. Navy, Model of 1928” was adopted.
Auto-Ordnance received an order for 500, which along with the 340 M1921 Thompsons acquired previously, was enough to satisfy the limited demands of the U.S. Navy and Marine Corps for the foreseeable future. The guns were from the original batch of 15,000 guns made by Colt in 1921. Indeed, the Colts were sufficient to meet the demand for Thompsons until the eve of World War II. The Model of 1921 marking had the “1” overstamped to “8”, but, except for the addition of a heavier actuator to reduce the rate of fire, the Cutts Compensator and the horizontal fore-end, the M1928 U.S. Navy Thompson retained the same features found on the commercial guns of the same era.
The U.S. Army had shown even less interest in the Thompson. By the late '20s, the Army was in the process of acquiring tanks and scout cars to equip the Cavalry and standardized Thompsons as “non-essential limited procurement.” Numbers were small, however, since the Cavalry preferred to wait for the Garand rifle still under development. The Infantry saw no need for Thompsons whatsoever.
When the M1 Garand rifle was finally adopted and issued, the Cavalry soon realized that the Thompson was better than the M1 rifle for its uses, given its more compact size and greater firepower. In September 1938, the Thompson changed from Limited Procurement status to Standard and was designated the “Submachine Gun, Caliber .45, Model of 1928A1.” Even with its official adoption, the Thompson was relegated to use as an auxiliary Cavalry arm, and little thought was given to its potential value as an infantry arm.
The M1928A1 Thompson was issued with the same type of 20-round box magazine and 50-round drum magazine as used with the M1921 and M1928 Navy models. A very heavy and cumbersome 100-round drum was produced previously for the commercial Thompsons, but none were procured by the government.
In June 1939, the Army ordered 950 Thompsons from Auto-Ordnance. The inventory of Colt-made guns was just about depleted, and Auto-Ordnance made plans to have M1928A1s produced under license by the Savage Arms Company plan in Utica, New York. Gambling on increased orders from the U.S. as well as other countries due to war breaking out in Europe, Auto-Ordnance wanted to have its own factory. An old brake-lining plant in Bridgeport, Connecticut, was acquired and renovated for armsmaking.
Auto-Ordnance guessed right about the demand for Thompsons. By late 1940 the Army had placed orders for 20,405 additional M1928A1s, and a year later the total ordered came to 319,000. The first Thompsons started to come off the Savage assembly line in February 1941, and the Bridgeport plant began to deliver guns by August of the same year. Many of the Thompsons ordered by the government were intended for allied nations, particularly Great Britain, under the “Lend-Lease” Program.
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.