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The Thompson Submachine Gun ID Guide, Part VI: The M1 Thompson Submachine Gun

By Frank Iannamico

Late in 1941, the Ordnance Department had several meetings with Auto-Ordnance officials and engineers to discuss the immediate need to increase production of submachine guns by nearly one-hundred percent. As a direct result of the meetings, subcontractor Savage Arms introduced the M1 Thompson just a few months later.

In November of 1941, the engineering staff at Savage Arms was already busy conducting a study of how the 1928A1 model Thompson could be simplified. The engineers were looking for ways to decrease the time it took to manufacture the gun. The area that consumed much of the manufacturing effort was the receiver, more specifically the rails inside of the receiver that the bronze lock traveled on. The three-piece bolt/lock/actuator assembly of the 1928 model was also labor intensive to manufacture. The Savage engineers doubted that the locking device was necessary for the relatively low power of the .45 caliber cartridge. There were many submachine guns being produced throughout Europe and none of them had required the use of any type of device to retard the blow-back action of the bolt.

In late February of 1942, a prototype of the Savage redesigned Thompson Submachine Gun was ready to be submitted to the Ordnance Department for testing and approval. The Savage Corporation had just conducted their own 10,000 round endurance test, which the new weapon successfully passed. The Savage Corporation told Auto-Ordnance that they were submitting the redesigned Thompson “without any claims for compensation, reimbursement, royalty or patent interest”. The new Thompson was sent to Aberdeen Proving Ground for testing and evaluation. The Aberdeen trials of the new Thompson were rather brief and concluded on 21 March 1942. The Aberdeen report stated that “The overall consideration of the mechanical functioning leads to the judgement that the gun as a mechanism is equivalent or superior to the M1928A1”. After a few government recommended alterations to the prototype were made, the new Thompson was recommended for adoption as Submachine Gun, Caliber .45 M1, at an Ordnance Committee meeting held on 24 March 1942. At the same meeting it was recommended that the 1928A1 be reclassified as Limited Standard. On 25 April 1942, the Ordnance Committee approved the recommendation for adoption of the M1 Thompson.

The Stevens Company’s chief designer Nicholas Brewer, and Savage engineer John Pearce were credited with designing the new Thompson. The M1 model had no provisions to use the drum style magazines. It used the same box style magazine as the 1928A1 model. Savage had estimated the cost of producing an M1 model to be $4.36 for material and $6.94 for labor with 175% in overhead. The total cost for Savage to manufacture an M1 would be $23.44. On 24 February 1942, Savage agreed to a contract to manufacture the M1 model for Auto-Ordnance at a cost of $36.37 per unit, providing Savage with a profit of $12.93 per gun. Auto-Ordnance in turn charged the U.S. Government $43.00 for an M1 model. The price varied slightly from contract to contract.

One of the major design differences between the M1 Thompson and the earlier 1928A1 model was that the M1 used a straight blowback design, eliminating the controversial bronze “H” lock. This allowed the receiver to be redesigned for easier manufacture. The bolt assembly was a very simple one-piece block of steel. The inside of the M1 upper receiver simply had a rectangular channel milled into it to accommodate the bolt. The internal felt oiler of the earlier 1928 model was considered unnecessary and thus eliminated in the M1 design. The cocking handle and slot were moved from the top of the receiver to the right side. An enlarged takedown notch was cut in the slot to allow the handle to be easily removed for disassembly (after the receiver has been separated from the frame). Since the drum style magazines had proven unsuitable for military use, the M1 series receivers were not designed to utilize them. The lateral grooves on the sides of the magazine well for accommodating the drum magazine were eliminated.

Like the 1928 and 1928A1, the early M1 Thompson used a separate firing pin and spring, that was the same as the ones used in the earlier models. A separate hammer was also used but the hammers for the M1 were configured differently than that of the 1928 and 1928A1 models. The 1928 series hammers were a triangular configuration, while the M1 hammers were crescent shaped.

Both the recoil spring pilot rod, and the buffer were redesigned for the M1 series Thompson receivers. The pilot rod for the recoil spring was simplified for easier manufacture, and was held in place by the new style buffer. The M1 buffer assembly consisted of a fibre plate secured between two metal plates. The buffer was a substantial improvement over the simple fibre disc used in the 1928 series Thompsons. The M1 pilot rod and buffer also made disassembly and assembly of the weapon much easier, and lessened the possibility of damaging the recoil spring in the process. The pilot rod could be removed and installed through the rear of the receiver once the buffer was removed.

The M1 featured a smooth, unfinned barrel that was similar to the one fitted to the late 1928A1 models, but the muzzle end was not threaded for fitting a compensator. A simple blade style milled sight was fitted and pinned to the muzzle in place of the compensator.

Early M1’s were assembled using the same uncheckered “paddle” style rocker and safety levers that were used on the U.S. 1928A1 model. The magazine latches on many M1 and M1A1 weapons were the same style as the 1928 and 1928A1 type that had a raised area for securing a drum magazine. Since the M1 series would not accommodate a drum style magazine, the magazine latch was later redesigned, and the raised area on the latch was eliminated.

The pistol grip was reconfigured to fit the redesigned M1 trigger frame, and the stock was redesigned as it was now semi-permanently attached directly to the frame with screws. The contour of the stock also differed from the earlier 1928 and 1928A1 design. The stock’s buttplate was also simplified, but retained a spring-loaded trap door for storage of a larger cylindrical oil bottle. The early M1 stock came from the factory without the reinforcing cross bolt installed. The cross bolts were not included until mid 1943 on the M1A1, although most M1 models had their stocks upgraded to include the cross bolt. The standard military horizontal style foregrip was installed on all M1s.

One of the distinctive features of both the M1 and M1A1 guns were their breech bolts, that had been redesigned to have two sear notches machined into them, so that the weapon’s safety could be applied when the bolt was forward on an empty chamber. The 1928 and 1928A1 bolts had only one notch machined into it, and the safety could not be applied when the bolt was closed. Several accidents had been reported with earlier 1928 and 1928A1 models when the weapon was carried with a loaded box style magazine and the bolt closed on an empty chamber. If the weapon was accidentally dropped on its butt, inertia would allow the bolt to travel rearward far enough to pick up a round from the magazine, but often not far enough to engage the sear (that would have held the bolt rearward). The bolt would begin to move forward and chamber the round. The result was usually an accidental discharge of the weapon.

Note; some very early M1 Thompson bolts were manufactured with only one sear notch. An Ordnance Department directive ordered that all one-notch bolts be deemed obsolete, and be replaced with the two-notch bolts as they became available.

While in July of 1942, Savage had turned out 48,000 M1 guns, Auto-Ordnance was struggling to meet its scheduled production mark. The assembly line conversion from the 1928A1 model to the M1 in July through October of 1942, caused serious production setbacks at the Auto-Ordnance plant in Bridgeport. Contributing to the production delays were problems in deliveries of materials, equipment and tooling authorized by the government for M1 production.

After the M1 production finally commenced at the Bridgeport plant, more problems were encountered. The Springfield Ordnance District refused to accept any of the Auto-Ordnance manufactured M1’s because their full-auto cyclic rate exceeded the 860 rounds per minute Ordnance Department specification. Officials from Washington, the District Ordnance office along with Auto-Ordnance engineers conducted studies and tests, all failing to provide a correction for the condition. Finally on 9 December 1942, official notice from the Ordnance Department’s office in Washington gave the district permission to waive the rate of fire requirement and accept the Bridgeport M1 guns. In the interim, identical M1 Thompsons being produced at Savage were being accepted in large quantities by the Rochester Ordnance District without any problems.

The rear sight that was first installed on the M1 was the same simple unprotected “L” style that was also installed on late manufacture 1928A1s. This design proved to have several flaws and was replaced by a similar sight, but with protective side ears added. On 29 June 1942, a letter was sent from Auto-Ordnance to Savage requesting a few changes in a contract originally dated 24 February 1942. One of the changes proposed was to the rear sight. Savage agreed to the changes as long as they conformed to Auto-Ordnance drawings 42-53 for the front sight and A42-86 (protected rear sight). Auto-Ordnance accepted the proposed supplement to the original contract on 17 July 1942. The new sight first appeared on the Savage Arms M1 Thompson in the high 137,000 serial number range. All of the Subsequent M1 and M1A1 models were fitted with new “protected” style sight. The drawing number for the new sight was B147717.

A few of the M1 side-protected style “L” sights have been noted on 1928A1 models depicted in Army manuals. Two such weapons were also documented in the author’s database. The protected rear sight was introduced after the 1928A1 production run had concluded. A protected type of “L” sight fitted to a 1928 model was most likely the result of the weapon going through an Ordnance Department rebuild program.

While Lyman manufactured most of the “L” style sights, Savage, Auto-Ordnance and the H.L. Judd Company manufactured the improved model with side protectors. Manufacturer’s markings noted on the second design sights are either AOC or J, but the majority of them appear to be unmarked. The rebuild guidelines of the Ordnance Department required that all rear sights without protective side ears would be replaced with rear sight drawing number B147717.

There were approximately 285,480 M1s manufactured by Savage Arms and Auto-Ordnance, Bridgeport. Savage Arms manufactured the vast majority of the early M1 model.

This article was excerpted in part from the book “American Thunder, the Military Thompson Submachine guns”. The book is now available from Moose Lake Publishing 207-683-2959


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