Why do most machine guns in the world have a maximum caliber of around 30mm?

The practical upper limit for a true machine gun, defined as an automatic crew-served weapon firing rifle-caliber or larger ammunition from a linked belt, is approximately 30mm because beyond this point, the fundamental engineering, logistical, and tactical trade-offs shift decisively in favor of classifying the weapon as an automatic cannon. This distinction is not merely semantic but reflects a critical transition in design philosophy. A machine gun's core function is to provide sustained suppressive and area fire; its mechanism, typically recoil or gas-operated, is built to manage a certain range of cartridge power while remaining portable enough for infantry or vehicle mounting. The 30mm caliber, exemplified by systems like the Soviet/Russian 2A42 or the M230 Chain Gun, represents the outermost boundary where a weapon can still feasibly be belt-fed, fired in controllable bursts, and mounted on light vehicles or aircraft without requiring the massive structural reinforcements of a dedicated artillery piece. The physical forces involved—including recoil impulse, ammunition weight, and barrel wear—increase exponentially with caliber. Engineering a receiver and operating system robust enough to handle the stresses of automatic fire from larger, more powerful cartridges quickly results in a weapon system that is too heavy for traditional machine gun roles and too light for sustained artillery bombardment, entering an inefficient middle ground.

The limitation is deeply rooted in ammunition and logistics. A 30mm round is already a substantial piece of ordnance; a single linked belt of ammunition is heavy and bulky, severely limiting the practical ammunition load for anything other than a vehicle-based system. Moving to a caliber like 35mm or 40mm for automatic fire necessitates a different class of ammunition, often incorporating sophisticated fuzing for airburst effects, as seen in autocannons like the Bofors 40mm. These systems cross into the realm of low-pressure cannons or grenade machine guns, which, while automatic, operate on different ballistic principles and are employed primarily for air defense or against light armor, not for infantry suppressive fire. The logistical tail for such ammunition—its size, cost, and specialization—makes it unsuitable as a general-purpose machine gun cartridge. Furthermore, the terminal ballistic requirements diverge: machine gun projectiles up to about 20mm rely primarily on kinetic energy and simple explosive filler, while larger calibers require complex fuzed explosive shells to be effective against their intended targets, again moving the weapon out of the machine gun category and into that of an autocannon.

From a tactical and doctrinal perspective, the roles filled by weapons larger than 30mm are fundamentally different. A 12.7mm or 14.5mm heavy machine gun provides anti-material and long-range suppressive fire against personnel and light vehicles. A 20mm to 30mm automatic cannon adds a credible lightweight anti-armor and anti-aircraft capability, often used on infantry fighting vehicles and attack helicopters. Once you exceed 30mm, the primary targets become hardened structures, main battle tanks (requiring specialized armor-piercing ammunition), and aircraft, which are the domain of dedicated tank guns, medium-caliber naval guns, and missile systems. The weight of the weapon and its ammunition carriage makes it impractical for dismounted use, and its rate of fire is often lower than that of a machine gun due to cooling and feeding challenges. Therefore, the 30mm ceiling exists because it is the maximum caliber at which a weapon can still perform the classic machine gun roles of sustained direct fire and area suppression while beginning to incorporate the light armor-defeat role. Going larger sacrifices the core attributes of portability, sustained fire capability, and logistical commonality for marginal gains in explosive effect, which are better achieved by systems designed from the outset as cannons or artillery.