[12] The M3 was chambered for the same .45 ACP round fired by the Thompson submachine gun, but was cheaper to mass produce and lighter, at the expense of accuracy.

M3A1 submachine guns were retired from U.S. frontline service after 1959, but continued to be issued, for example as backup weapons for armored vehicle crews as late as the Gulf War (1990–1991).

[13] The Ordnance Department requested the Army submit a list of requirements for the new weapon, and Ordnance in turn received a separate list of requirements from both the Infantry and Cavalry branches for a shoulder-fired weapon with full or semiautomatic fire capability in caliber .45 ACP or .30 Carbine.

[12] The T20 was formally approved by U.S. Army Ordnance for production at GM's Guide Lamp Division in Anderson, Indiana, in December 1942 as the U.S. Submachine Gun, Caliber .45, M3.

Additionally, Rock Island Arsenal and Buffalo Arms Corporation manufactured parts for a limited number of 9 mm conversion kits for the M3.

[19][20] As such, replacement parts, weapon-specific tools, and sub-assemblies were not made available to unit-, depot-, or ordnance-level commands at the time of the M3's introduction to service.

[21][22] In 1944, a shortage of M3 submachine guns created by the need for interim production changes forced U.S. Army Ordnance workshops to fabricate pawl springs and other parts to keep existing weapons operational.

[12] An investigation revealed several deficiencies in the construction of the M3's bolt retraction mechanism, together with issues concerning barrel removal and retention as well as easily bent rear sights.

[12] After new complaints were raised about accidental magazine releases and failure of the wire buttstock to remain in place in the collapsed position, two additional changes were made to M3 production and approved by Ordnance on 31 August 1944.

[12] This included a small sheet metal guard around the magazine release button, and the inclusion of a stop between the two rods forming the wire stock at the butt end.

The M3 was also ideal for the Pacific War because the Thompson could easily jam if not cleaned, which had to be done constantly in the jungle environment because the action did not have a cover over the ejector as the M3 did.

However, due to unforeseen production delays and requests for modifications, the M3 was introduced later than expected, and purchases of the Thompson continued until February 1944.

[27] Within a year, the M3A1 had been replaced by the 9 mm Heckler & Koch MP5 submachine gun in Delta Force use, but a few were kept past that date as it was felt that the M3A1 performed better with a suppressor than the MP5.

The bolt was drilled longitudinally to support two parallel guide rods, upon which were mounted twin return (recoil) springs.

This configuration allows for larger machining tolerances while providing operating clearance in the event of dust, sand or mud ingress.

It proved popular in combat, as frequent night engagements emphasized the need to reduce flash signatures on small arms.

[citation needed] Projecting to the rear is a one-piece wire stock made from a formed steel rod that telescopes into tubes on both sides of the receiver.

[32] The M3's cocking handle assembly is located on the right-hand side of the receiver on the ejector housing, just forward and above the trigger, and consists of nine parts.

With receiver walls made of relatively thin-gauge sheet metal, the M3/M3A1 is subject to disabling damage if dropped on an open dust cover—the covers bend easily, negating the safety feature.

[33][34] Unlike the Thompson, the M3 feeds from a double-column, single-feed detachable box magazine which holds 30 rounds and was patterned after the British Sten magazine; the single-feed design proved difficult to load by hand, and is more easily jammed by mud, dust, and dirt than double-column, staggered-feed designs like the Thompson.

[12] The M3A1 modifications resulted in a more reliable, lighter weight, easier to maintain, and easier to field strip submachine gun; the original M3 needed both the trigger guard removed and the cocking crank assembly detached from the receiver housing before unscrewing the barrel, but the M3A1 only required the user unscrew the barrel.

[16] Because it had already been issued in large numbers, the existing M3 magazine design was retained, despite demonstrated deficiencies exposed during the weapon's firing trials and its early combat service.

[34] In an effort to improve reliability, a hard plastic Tenite cap designated T2 was adopted in November 1944 to fit over the feed lips of loaded magazines.

[38] Initially, M3 submachine guns returned for repair were not upgraded to the M3A1 standard, but merely inspected to ensure they had the improved M3 housing assembly and magazine release shield.

[41] These incidents were sometimes caused by dropping the weapon on a hard surface with an impact sufficient to knock open the ejection port cover and propel the bolt backwards (but not enough to catch the sear).

[41][42] In 1945, the Guide Lamp factory manufactured 15,469 M3A1 submachine guns before production contracts were canceled with the end of the war.

[46][47] Constructed of somewhat thinner-gauge steel than the U.S. M3A1, the PAM-1 was in essence a 7/8-scale replica of the U.S. weapon in 9 mm Parabellum caliber,[48] but was lighter[49] and had a higher rate of fire.

Problems with accidental discharges and accuracy with the PAM-1 led to an improved selective-fire version with a grip safety on the magazine housing known as the PAM-2, first introduced in 1963.

A number of PAM-1 and PAM-2 submachine guns were used by the Argentine Army during the Falkland Islands War with the United Kingdom in 1982, and captured examples were tested by British military forces.