The choice of the M240 (7.62mm) as the coaxial machine gun on the M1 Abrams tank instead of the heavier M2 (.50 caliber) is based on a combination of logistical, operational, and design considerations. Here’s a breakdown of the reasons behind this decision:
1. Role and Target Engagement:
- Primary Role: The coaxial machine gun is intended for engaging infantry, light vehicles, and soft targets at ranges where the tank’s main gun is not necessary. The M240 (7.62mm) is highly effective against these types of targets.
- Ammunition Volume: The M240 allows the tank to carry a significantly larger ammunition supply compared to a heavier M2 machine gun. Given that the coaxial gun is often used for suppression and support, having a large ammo reserve is crucial. The M240 can carry thousands of rounds internally, which is ideal for its role in prolonged engagements.
2. Space and Weight Constraints:
- Turret Space: Installing a .50 caliber M2 as a coaxial weapon would require more space due to its size and the larger ammunition. The M240 is more compact, fitting more easily into the tight space within the Abrams’ turret.
- Weight Considerations: The M2 is significantly heavier than the M240. Using the M2 as a coaxial would add to the turret’s weight, potentially affecting the tank’s maneuverability, balance, and the precision of the stabilized gun system. The M240’s lighter weight makes it a better match for the coaxial role, which emphasizes volume of fire over heavier firepower.
3. Rate of Fire and Suppression Capability:
- The M240’s rate of fire (around 650-950 rounds per minute) provides an excellent rate of sustained fire for suppression, which is critical when engaging enemy infantry or lightly protected positions.
- Balance Between Firepower and Ammunition Economy: The M240 offers a balance between being powerful enough to deal with infantry while not consuming ammunition as rapidly as the M2. This makes it a more efficient choice for the tank’s internal armament, reducing the need for frequent reloads in combat.
4. Historical Doctrine and Adaptations:
- Historically, coaxial machine guns in Western tanks, including the M1 Abrams, have been in the 7.62mm range, like the M1919 Browning during World War II and the M240 since its adoption. This reflects a doctrinal preference for lighter, versatile coaxial guns paired with heavier external guns for specialized roles.
- External M2 for Flexibility: The Abrams is equipped with an M2 .50 caliber machine gun mounted on the turret top (now often remotely operated with systems like CROWS). This allows the crew to use the .50 caliber when needed, such as against lightly armored vehicles or low-flying aircraft, without the drawbacks of mounting it internally.
5. Technology Advancements: Rangefinding and Fire Control:
- Modern Rangefinding: In the past, machine guns were sometimes used for rangefinding (spotting rounds). However, with the advent of advanced laser rangefinders and digital fire control systems, the need to use machine gun tracers for distance estimation has largely disappeared.
- The Abrams’ sophisticated fire control system means that the M240 is used more for area suppression and engaging exposed targets rather than precise spotting for the main gun.
6. Design Choices and Tank Doctrine Variations:
- Other nations have made different choices based on their design philosophies. For example, the French AMX-30 and early Leclerc tanks used larger caliber coaxial weapons, such as 20mm autocannons, but this came with trade-offs in terms of ammunition capacity and turret space.
- The U.S. Army’s approach emphasizes a combination of volume of fire, logistical simplicity, and adaptability. The M240 strikes a balance between being effective for close-range suppression without the space and weight penalties of a larger caliber machine gun.
Summary: Why the M240 Fits the Coaxial Role
The decision to use the M240 as a coaxial machine gun on the M1 Abrams is about balancing the need for sustained firepower, space management within the tank, and the ability to engage the most likely threats efficiently. While the M2 offers superior firepower, it comes with downsides in terms of size, weight, and ammo storage that make it less ideal for internal mounting. The M240, by contrast, offers a high rate of fire and the ability to carry more rounds, which aligns well with the Abrams’ needs in various combat scenarios. The presence of the M2 externally gives tank crews the option to engage tougher targets when necessary, while the M240 remains the workhorse for engaging infantry and lighter threats.