Submachine gun
A submachine gun is a lightweight, portable automatic firearm chambered for pistol-type ammunition and typically fired from the shoulder or hip.[1] Designed for close-quarters combat, it employs a blowback or similar simple mechanism to achieve high rates of sustained fire from detachable magazines, distinguishing it from rifles by its use of lower-powered handgun cartridges that enable reduced recoil and compactness.[2][3] Emerging during World War I amid the static demands of trench warfare, submachine guns addressed the need for rapid, suppressive firepower in confined spaces where full-powered rifles proved cumbersome.[3][4] The German MP 18, introduced in 1918, marked the first widespread deployment of such a weapon, influencing subsequent designs by prioritizing volume of fire over precision at range.[5] Their role expanded dramatically in World War II, equipping infantry, paratroopers, and vehicle crews with models like the American Thompson and Soviet PPSh-41, which enhanced squad-level maneuverability and offensive capability in urban and forested environments.[6] Postwar developments shifted toward personal defense weapons, yet submachine guns persist in special operations and law enforcement for their reliability in high-threat, low-signature scenarios, though challenged by advancements in intermediate cartridges that offer superior ballistics.[3][7]Definition and Classification
Terminology and Etymology
The term "submachine gun" originated in the United States in the early 1920s, coined by Brigadier General John T. Thompson to describe a new class of compact, fully automatic firearms chambered for handgun cartridges, intended for close-quarters use by infantry to overcome trench stalemates.[5] Thompson, who led the Auto-Ordnance Corporation's development of the weapon patented in 1920, used the designation to differentiate it from belt-fed, crew-served machine guns like the Maxim, emphasizing its lighter weight (around 10 pounds unloaded for the early Thompson Model 1921) and one-handed or shouldered firing capability.[8] Etymologically, "submachine gun" derives from the prefix "sub-," indicating a subordinate or scaled-down variant of the "machine gun," which since the late 19th century had referred to sustained-fire weapons typically using rifle-caliber ammunition and requiring sustained bipod or tripod support.[9] The earliest documented English usage appears in 1920, in a Wisconsin newspaper reference predating widespread adoption but aligning with Thompson's promotional efforts.[10] This nomenclature reflected the weapon's mechanical similarity to machine guns—via blowback or recoil operation enabling automatic fire—but its reduced scale and pistol cartridge chambering (e.g., .45 ACP in the Thompson), which limited effective range to under 200 yards while prioritizing controllability in bursts.[2] Internationally, equivalent terms emerged concurrently, often translating to "machine pistol" to highlight the handgun ammunition and ergonomics akin to enlarged pistols with stocks. The German Bergmann MP 18, issued in 1918 as the first production submachine gun, was officially designated Maschinenpistole 18 ("machine pistol 18"), a term rooted in its 9mm Parabellum chambering and trench-assault role with Sturmtruppen units.[5] Similar phrasing appeared in French as pistolet-mitrailleur and in Russian as pistolet-pulemyot, underscoring the conceptual blend of pistol portability and machine-gun automaticity, though English usage standardized on "submachine gun" post-World War I due to Thompson's influence and U.S. manufacturing prominence.[9] By the 1930s, the term had globalized in military doctrine, as seen in British adoption of "machine carbine" for weapons like the Sten but eventual convergence on submachine gun equivalents.Core Characteristics
A submachine gun is a selective-fire firearm that fires pistol cartridges from a detachable box magazine, combining the automatic fire capability of a machine gun with the compact form factor of a handgun for use in close-quarters combat.[3] These weapons are distinguished by their use of handgun ammunition, such as 9×19mm Parabellum or .45 ACP, which produces lower recoil compared to rifle cartridges, enabling higher rates of fire and better controllability during sustained bursts.[3] Most submachine guns operate via a blowback mechanism, where the bolt is driven rearward by the pressure of the expanding gases pushing on the cartridge case, cycling the action without a locked breech for simplicity and reliability in adverse conditions.[11] Some designs, like the Heckler & Koch MP5, incorporate delayed blowback systems, such as roller-delayed, to enhance safety and reduce bolt velocity.[12] Typical cyclic rates range from 600 to 900 rounds per minute, with effective ranges limited to 100-200 meters due to the ballistics of pistol rounds.[13] Submachine guns feature short barrels, often 150-250 mm in length, and overall weights of 2-4 kg, prioritizing portability for vehicle crews, special forces, or urban operations over long-range accuracy.[3] This design allows for shouldered firing with stocks or braces, setting them apart from machine pistols, which lack such features and are fired one-handed like large handguns.[14] Unlike personal defense weapons, which may use specialized intermediate cartridges to defeat body armor, submachine guns rely on standard pistol calibers for volume of fire rather than penetration.[15]Distinctions from Related Firearms
Submachine guns are distinguished from full-power machine guns primarily by their use of pistol-caliber ammunition, lighter construction, and employment of box magazines rather than belts or drums for sustained fire, enabling greater portability for close-quarters use rather than suppressive fire at extended ranges.[14] Machine guns, such as the M60 or MG42, chamber rifle cartridges like 7.62x51mm NATO, weigh significantly more (often exceeding 10 kg unloaded), and incorporate heavier barrels with quick-change mechanisms to manage heat from prolonged automatic fire, whereas submachine guns prioritize rapid deployment over endurance, with effective ranges typically limited to 50-100 meters due to lower-velocity projectiles.[16][17] In contrast to assault rifles, submachine guns fire handgun cartridges (e.g., 9x19mm Parabellum) that lack the ballistic performance for engagements beyond 150 meters, rendering them ineffective against personnel at rifle distances where assault rifles excel with intermediate cartridges like 5.56x45mm NATO, which provide flatter trajectories and greater penetration.[18] Assault rifles, exemplified by the M16 or AK-47 variants, feature longer barrels (typically 14-20 inches) and rifle stocks for stability during semi-automatic aimed fire, whereas submachine guns emphasize compact designs with barrels under 10 inches for maneuverability in confined spaces like urban combat or vehicle operations.[14] Machine pistols differ from submachine guns in their handgun form factor, lacking a dedicated shoulder stock and relying on pistol grips for one- or two-handed firing, which compromises controllability during full-automatic bursts compared to the stabilized platform of submachine guns equipped with stocks or braces.[14] Examples include the Glock 18, with a 4.5-inch barrel and 17-33 round capacity, versus submachine guns like the MP5, which adds a folding stock and longer barrel for improved accuracy and reduced muzzle climb.[19] This distinction arises from design intent: machine pistols serve as secondary or concealed weapons, while submachine guns function as primary shoulder-fired arms.[20] Personal defense weapons (PDWs), such as the FN P90 or MP7, overlap in role with submachine guns as compact automatic firearms for non-frontline personnel but diverge in ammunition, employing specialized bottlenecked cartridges (e.g., 5.7x28mm or 4.6x30mm) optimized for body armor penetration at velocities approaching rifle rounds, unlike the standard pistol calibers of submachine guns that offer limited barrier defeat.[21] PDWs emerged in the 1990s per NATO requirements for rear-echelon defense against Soviet-style body armor, prioritizing low weight and high capacity (e.g., 50 rounds in the P90's bullpup configuration) over the traditional submachine gun's focus on sheer volume of low-energy fire.[22][15]Historical Development
Origins in Early 20th Century
The submachine gun emerged during World War I as a response to the static trench warfare on the Western Front, where soldiers required a lightweight, automatic firearm for close-quarters assaults beyond the capabilities of pistols or rifles. Early experiments focused on adapting pistol-caliber ammunition for rapid fire in portable designs. In 1915, Italian engineers at Officine Villar Perosa developed the Villar Perosa M1915, a twin-barreled 9mm Glisenti automatic weapon firing at approximately 1,200 rounds per minute per barrel, initially intended for aircraft use but later adapted with a stock for infantry by Arditi shock troops.[23] Though innovative, its awkward dual-barrel configuration and lack of a conventional shoulder stock limited its practicality as a handheld weapon, marking it more as a precursor than a fully realized submachine gun.[24] Concurrent Austro-Hungarian efforts yielded the Standschütze Hellriegel M1915 prototype, a water-cooled design chambered in 9mm Steyr pistol rounds, featuring a drum magazine and aimed at providing suppressive fire in trenches. Only a handful of prototypes were produced in 1915, and testing revealed reliability issues, including barrel ruptures, preventing adoption or mass production.[25] These early attempts highlighted the engineering challenges of managing heat and recoil in compact automatic systems but remained experimental and uninfluential on broader development. The first practical submachine gun arrived with the German Bergmann MP18/I, designed by Hugo Schmeisser and Theodor Bergmann in late 1917, entering production in early 1918. Chambered for the 9x19mm Parabellum cartridge, it employed a simple blowback mechanism with a cyclic rate of about 500 rounds per minute, a wooden stock, and a 32-round snail drum magazine, enabling effective use by Sturmtruppen in storming trenches during the Spring Offensive.[26] Approximately 30,000 units were manufactured by war's end, demonstrating the weapon's viability despite the Treaty of Versailles' subsequent restrictions on German arms production. This design directly influenced interwar submachine guns, establishing the core principles of pistol-caliber automatic fire from the shoulder.World War I and Immediate Aftermath
The Bergmann MP 18, developed in early 1918 by Hugo Schmeisser at Bergmann Waffenfabrik, represented the first practical submachine gun designed specifically for infantry use in trench warfare. Chambered for the 9×19mm Parabellum cartridge and utilizing a simple blowback operation, it fired from an open bolt at approximately 500 rounds per minute, with an effective range of 100 meters. The weapon adapted the 32-round snail drum magazine from the Dreyse MP 1910 machine pistol, addressing the need for suppressive fire in close-quarters assaults where bolt-action rifles proved inadequate. Approximately 50,000 units were ordered by the German Army, though only about 17,677 were produced before the war's end.[27][28] Introduced to Sturmtruppen (assault troops) during the German Spring Offensive of March 1918, the MP 18 enabled rapid advances through enemy trenches by providing volume of fire superior to pistols or rifles in confined spaces. Despite its late deployment—limited to the war's final months—accounts from German forces highlighted its effectiveness in storming positions, though reliability issues arose from the drum magazine's tendency to jam under muddy conditions. Its combat debut influenced tactical shifts toward mobile assault groups, but the weapon's overall battlefield impact remained marginal due to production constraints and the Armistice on November 11, 1918. No comparable submachine guns were fielded by Allied forces during the conflict.[29][30] Following the war, the Treaty of Versailles explicitly banned German military possession and production of submachine guns, permitting only limited numbers for police use—reportedly around 300 units initially. Surplus MP 18s, however, circulated through exports and black-market channels, arming Freikorps paramilitaries in suppressing leftist revolts in Germany during 1919–1920 and supporting interventions in the Baltic states, such as the Latvian War of Independence. This proliferation exposed the design's principles to foreign militaries, prompting evaluations and unlicensed copies in countries like Finland and China, while underscoring the Versailles restrictions' limitations in preventing technological diffusion. The MP 18's legacy thus bridged World War I tactics to interwar innovations, demonstrating the submachine gun's potential for urban and irregular warfare despite regulatory curbs.[31][32]Interwar Period
The interwar period witnessed refinements to World War I-era designs and the emergence of new submachine guns, driven by lessons from trench warfare and evolving tactical doctrines favoring close-quarters firepower, though adoption remained limited by costs, treaties, and infantry reliance on rifles. Germany's MP18 influenced subsequent models, with Hugo Schmeisser's MP28/II, produced from 1928 by C.G. Haenel, incorporating a 32-round vertical box magazine and improved ergonomics over the snail-drum-fed predecessor; restricted by the Treaty of Versailles, it saw export to China and use by police and border guards.[33][34] In the United States, General John T. Thompson's design, patented in 1918 and first produced in 1921 as the Model 1921 by Auto-Ordnance Corporation, fired .45 ACP from 20- or 50-round magazines at 800 rounds per minute; marketed initially to the military as a "trench broom," its high $200 price led to rejection, but it equipped the U.S. Marine Corps in small numbers and gained infamy with Prohibition-era gangsters, prompting law enforcement adoption like the 1920 U.S. Postal Inspection Service purchase.[35][36] European militaries accelerated development in the 1930s amid rearmament. Finland adopted Aimo Lahti's Suomi KP/-31 in 1931, a 9mm blowback weapon with a 71-round double-drum magazine, quick-detachable barrel, and bipod for sustained fire, producing over 80,000 by 1939 for elite troops due to its reliability in cold climates.[37] The Soviet Union introduced Vasily Degtyaryov's PPD-34 in 1935, chambered in 7.62x25mm Tokarev with a 71-round drum, assigning initial batches to NKVD border guards before wider trials.[38] Italy's Beretta Modello 38, adopted in 1938, offered selective fire in 9mm Glisenti with a 20- or 40-round magazine, emphasizing simplicity and issued to assault troops.[39] Britain pursued prototypes like the Lanchester (derived from MP28) but rejected mass production until wartime needs, reflecting doctrinal conservatism.[40] These weapons influenced mechanized infantry tactics, yet most armies procured in modest quantities—e.g., U.S. Army standardization of the Thompson in September 1938 yielded only 15,000 by 1941—prioritizing machine guns for suppression.[41]World War II Proliferation
The proliferation of submachine guns during World War II marked a pivotal expansion in their military adoption, driven by the demands of close-quarters battle in urban environments, trench systems, and rapid assaults, particularly evident in the Eastern Front and Pacific theaters. Major powers prioritized mass production of simplified designs using stamped metal components to achieve economies of scale amid resource constraints. The Soviet Union exemplified this trend with the PPSh-41, which entered production in 1941 and reached approximately 6 million units by war's end, facilitating widespread issuance to infantry squads for high-volume suppressive fire in human-wave tactics.[42][43] Germany's MP 38/40 series, introduced in 1938 and refined for wartime output, yielded about 1.1 million units, equipping non-commissioned officers, paratroopers, and Panzergrenadier assault teams despite higher per-unit costs compared to later Allied stamped-metal alternatives.[44][45] Britain's Sten gun, developed in 1941 as a low-cost response to home defense needs and Lend-Lease shortages, epitomized wartime improvisation with its minimalist tube-and-stamp construction, resulting in over 4 million produced across variants by 1945. This enabled arming resistance forces, commandos, and Commonwealth troops effectively, though early models suffered reliability issues from rushed manufacturing. The United States initially relied on the Thompson submachine gun, with over 1.5 million military models manufactured during the war after 1942 adoption of simplified M1 and M1A1 variants, but shifted to the cheaper M3 "Grease Gun" from 1943, producing around 600,000 units to supply paratroopers, vehicle crews, and Marines in Pacific island-hopping campaigns.[46][47][48] Other Axis and Allied nations contributed to the global spread, though on smaller scales; Italy's Beretta Model 38, a pre-war design, saw limited production of about 100,000 units for elite Bersaglieri units, while Japan's Type 100, chambered in 8mm pistol rounds, numbered fewer than 10,000 due to industrial limitations and doctrinal preference for rifles. This era's output—totaling tens of millions across combatants—reflected a tactical evolution toward firepower density at squad level, supplanting bayonet charges and bolstering defensive positions, yet post-war analyses noted vulnerabilities like limited range and poor accuracy beyond 100 meters, influencing the rise of assault rifles.[49]Cold War Advancements
 During the Cold War era (approximately 1947–1991), submachine guns transitioned from mass infantry weapons toward specialized roles in close-quarters battle (CQB), counter-terrorism, and personal defense for vehicle crews and special forces, driven by the need for compact, controllable firearms amid proxy conflicts like the Korean War (1950–1953) and Vietnam War (1955–1975). Western designs emphasized reliability in adverse conditions and export potential, while Eastern Bloc nations largely extended World War II-era models such as the Soviet PPSh-41 and PPS-43, which saw continued production and use by allies, though innovation waned as assault rifles like the AK-47 gained primacy. Advancements included telescoping bolts for reduced length, delayed blowback mechanisms for improved accuracy over simple blowback, and standardization on 9×19mm Parabellum for logistics compatibility.[50][51] The Israeli Uzi, developed by Uziel Gal and prototyped in 1950, exemplified early Cold War innovation with its telescoping bolt design that shortened overall length to 650 mm while maintaining a 32-round magazine capacity and cyclic rate of 600 rounds per minute. Adopted by the Israel Defense Forces in 1951 and entering full production by 1954, the Uzi's stamped metal construction enabled cheap mass production, leading to widespread export to over 90 countries and adoption by U.S. special operations units during Vietnam. Its open-bolt, blowback operation proved robust in desert and urban environments, influencing subsequent compact SMG designs.[52][53] In West Germany, Heckler & Koch introduced the MP5 in 1966, adapting the roller-delayed blowback system from the G3 rifle to a 9mm platform, which reduced recoil and muzzle climb for better control during full-auto fire, achieving effective ranges up to 200 meters with a 600–800 rpm rate. Weighing 2.5 kg unloaded with a 30-round magazine, the MP5's precision and modularity— including suppressors and optics rails—made it ideal for police and counter-terror units, with variants like the MP5K (1976) further miniaturized for concealed carry. Adopted by over 40 nations' forces, including the West German GSG 9, it set standards for CQB accuracy.[54][55] The Czechoslovak Škorpion vz. 61, designed in 1959 for non-infantry use, advanced the machine pistol concept with a folding metal stock, 7.65×17mm Browning (.32 ACP) chambering for low recoil, and a 20-round magazine in a 3.1 kg package firing at 850 rpm. Intended for security personnel and tank crews, its compact 539 mm length and simple blowback mechanism facilitated export to Eastern Bloc allies and third-world insurgencies, foreshadowing personal defense weapon trends despite limited Soviet adoption of new SMGs post-1950s. Meanwhile, the U.S. Ingram MAC-10 (developed 1964) prioritized extreme portability (279 mm suppressed length) and high 1,000+ rpm rate for special operations, though its .45 ACP or 9mm blowback design suffered controllability issues, restricting it to niche roles.[56][57][58]Late 20th Century Transitions
In the 1970s, submachine gun designs emphasized extreme compactness and suppressibility to suit close-quarters and covert operations, exemplified by the Ingram MAC-10, developed in 1964 and entering production around 1970 in .45 ACP caliber with a cyclic rate exceeding 1,000 rounds per minute.[58][59] This open-bolt, blowback-operated weapon featured a telescoping bolt for reduced length and was marketed for military and law enforcement use, though its high recoil and controllability issues limited widespread adoption.[60] The Heckler & Koch MP5, introduced in 1966 but achieving peak prominence in the 1970s and 1980s, marked a shift toward precision and reliability in submachine guns via its roller-delayed blowback system, which improved accuracy over traditional blowback designs.[61] Adopted by German special forces in 1966 and gaining international acclaim during the 1980 SAS Iranian Embassy siege, the MP5 became the standard for counter-terrorism units worldwide, including U.S. SWAT teams and FBI HRT by the 1990s, due to its controllability in full-auto fire and adaptability with variants like the suppressed MP5SD.[62][63] By the late 1980s, military doctrines evolved amid rising body armor proliferation, prompting NATO to issue requirements in 1989 for a personal defense weapon (PDW) to supplant 9mm submachine guns for rear-echelon personnel, prioritizing armor penetration over volume of fire.[15] This led to innovations like FN Herstal's P90, developed from the late 1980s and chambered in the new 5.7x28mm cartridge for enhanced velocity and barrier defeat, signaling a transition from pistol-caliber submachine guns to compact, high-velocity alternatives better suited to modern threats.[64] Concurrently, submachine guns experienced a broader decline in frontline military roles during the late 20th century, as lighter, longer-ranged assault rifles like the M16 and AK-74 fulfilled versatile infantry needs, relegating SMGs to niche applications in special operations, vehicle crews, and urban policing where compact size and rapid fire remained advantageous.[65][7] This shift reflected causal factors including improved rifle ergonomics, reduced emphasis on mass close-combat assaults post-Vietnam, and the inefficacy of pistol rounds against emerging protective gear.[66]21st Century Evolutions
In the 21st century, submachine guns have largely transitioned from standard military issue to specialized personal defense weapons (PDWs) for roles such as vehicle crews, special operations, and close-quarters battle (CQB) in law enforcement, driven by advancements in carbine designs that offer superior range and penetration with intermediate cartridges.[3][67] The PDW concept, originating from a 1980s NATO requirement for compact firearms capable of defeating body armor beyond 9mm pistol rounds, emphasized high-velocity, low-recoil ammunition in lightweight platforms.[67][22] Key developments include the FN P90, introduced in 1990 after development from 1986, which fires the 5.7x28mm cartridge from a bullpup configuration with a 50-round top-mounted magazine, achieving effective penetration against Level IIIA armor at distances up to 200 meters.[68][69] Similarly, the Heckler & Koch MP7, entering production in 2001, utilizes the 4.6x30mm round in a gas-operated, polymer-framed design weighing approximately 1.9 kg unloaded, prioritizing controllability and modularity for suppressors and optics.[70] These PDWs represent an evolution toward integrated systems with reduced signature and enhanced ergonomics, though widespread adoption stalled as militaries favored versatile carbines like the M4 for logistics simplicity and ballistic performance against modern threats.[15] Contemporary evolutions focus on modularity and materials, with examples like the B&T APC9K Pro, a 9mm select-fire SMG fielded by the U.S. Army in 2022 for VIP protection in quantities of 315 units, featuring pistol-caliber compatibility with AR-pattern accessories for rapid reconfiguration.[71] Innovations such as delayed blowback systems, as in the KRISS Vector (introduced 2009), mitigate recoil through redirection of gases, enabling stable full-auto fire in compact forms suitable for urban operations.[72] Despite these refinements, submachine guns remain niche due to pistol ammunition's limitations against body armor and the dominance of short-barreled rifles, confining their use primarily to non-infantry personnel requiring concealability over extended engagement capability.[3][73]Design and Technical Features
Operating Mechanisms
Submachine guns primarily utilize blowback operating mechanisms, which harness the rearward momentum of the expanding gases propelling the cartridge case to cycle the bolt without employing a locked breech, a design enabled by the modest chamber pressures of pistol-caliber cartridges typically below 35,000 psi.[74] This contrasts with recoil-operated or gas-operated systems more common in rifles, as the short barrels and lower-powered ammunition of submachine guns render locked-breech designs unnecessarily complex and costly for mass production.[75] Simple blowback predominates in early and wartime models due to its mechanical simplicity, requiring only a heavy bolt mass and strong recoil spring to delay breech opening sufficiently for safe extraction, achieving cycle rates often exceeding 600 rounds per minute.[76] In the basic simple blowback configuration, the bolt remains open or closed prior to firing but relies on inertia and spring tension to resist initial gas pressure; upon ignition, the cartridge case drives the unlocked bolt rearward, extracting and ejecting the spent casing while compressing the spring to strip and chamber the next round from the magazine.[74] Open-bolt firing, prevalent in full-automatic submachine guns like the British Sten Mk II (produced from 1941 with a cyclic rate of 500 rounds per minute), positions the bolt rearward until the trigger releases it to chamber and fire, reducing cook-off risks from residual heat in sustained fire but potentially introducing slight accuracy variations from bolt velocity.[77] Closed-bolt variants, such as later semi-automatic conversions, offer improved trigger consistency and single-shot precision by holding a round in the chamber ready for firing, though they demand additional safety features to mitigate accidental discharges.[12] Delayed blowback mechanisms address limitations of simple blowback, such as excessive bolt mass needed for high-pressure loads or uncontrollable cyclic rates, by incorporating mechanical retardation to prolong breech closure until pressures normalize, often enabling lighter components and reduced felt recoil.[76] Roller-delayed blowback, patented in variations since the late 19th century but practically implemented in submachine guns post-World War II, employs cylindrical rollers that wedge into a barrel extension trunnion, creating mechanical disadvantage to slow bolt carrier acceleration; upon sufficient rearward force, the rollers cam inward, unlocking the system for extraction, as seen in the Heckler & Koch MP5 introduced in 1966 with a 800-rounds-per-minute rate and compatibility with 9mm Parabellum pressures up to 35,000 psi.[75][78][12] Lever-delayed blowback, another variant using a pivoting lever to multiply resistance against bolt movement, appears in designs like the Danish M/45 chambered in 9mm, where the lever's fulcrum delays opening by forcing accelerated mass displacement, enhancing controllability in compact platforms.[79] These advanced systems proliferated in the Cold War era for special forces applications, prioritizing accuracy and reliability over wartime simplicity, though simple blowback persisted in cost-sensitive designs due to fewer failure-prone parts.[80] Gas-operated submachine guns remain exceptional, limited to specialized prototypes like the American ArmaLite AR-57 (1960s) adapting short-stroke pistons, as pistol cartridges generate insufficient gas volume from stubby barrels for efficient operation without excessive complexity.[76]Ammunition and Caliber Choices
Submachine guns are designed to fire pistol cartridges rather than full-powered rifle rounds, prioritizing controllability during sustained automatic fire in close-quarters scenarios. This choice stems from the need for low recoil impulse, which allows for higher rates of fire—often exceeding 600 rounds per minute—without excessive muzzle climb or shooter fatigue in compact weapons weighing under 4 kilograms. Pistol calibers enable simpler blowback-operated mechanisms, reducing manufacturing complexity and costs compared to gas-operated rifle systems.[81][82] The 9×19mm Parabellum emerged as the dominant caliber with the Bergmann MP18 in 1918, selected for its moderate recoil, availability in European militaries, and compatibility with existing pistol ammunition supplies. Adopted widely in World War II models like the German MP40 and British Sten, it offered a balance of velocity (around 350-400 m/s) and magazine capacity, typically 30-50 rounds, suitable for trench and urban combat ranges under 100 meters. In contrast, the American Thompson submachine gun, entering production in 1919, chambered the .45 ACP cartridge for superior stopping power against unarmored targets, with its larger 11.43mm bullet delivering greater energy transfer despite lower velocity (about 260 m/s) and reduced capacity (20-50 rounds). This caliber choice reflected U.S. doctrine emphasizing individual marksmanship and one-shot stops in World War I trials.[82][83] Post-World War II designs continued favoring pistol calibers for logistical commonality with service pistols, such as the Israeli Uzi (9mm, 1950s) and German MP5 (9mm, 1966), which benefited from reduced training requirements and lighter weight—often half that of rifle-caliber alternatives. The .40 S&W and 10mm Auto appeared in 1980s-1990s models like the Heckler & Koch MP5/40, offering intermediate ballistics between 9mm and .45 ACP for enhanced penetration without prohibitive recoil. However, proliferation of body armor prompted specialized personal defense weapon (PDW) calibers, such as the 4.6×30mm in the HK MP7 (1999) and 5.7×28mm in the FN P90 (1990), designed for better armor defeat at pistol-like velocities while maintaining low recoil. These represent a minority deviation, as traditional pistol calibers persist for their proven efficacy in non-armored threats and supply chain advantages.[81][84]| Caliber | Typical Velocity (m/s) | Key Advantages | Example SMGs |
|---|---|---|---|
| 9×19mm Parabellum | 350-400 | High capacity, low recoil, widespread availability | MP18, MP5, Uzi |
| .45 ACP | 250-280 | Superior stopping power, energy dump | Thompson, MAC-10 |
| .40 S&W | 300-350 | Balanced penetration and expansion | MP5/40, UMP |
| 5.7×28mm | 700+ | Armor penetration, flat trajectory | FN P90 |