FIREPOWER/LETHALITY

The M18 carried the 76.2mm M1A1 L/52 gun with a slightly larger bore than the KwK 42 L/70 mounted on the Panther but with inferior performance. The M1A1 fired M62A1 armor-piercing capped-tracer (APC-T) rounds that were ineffective against the Panther’s frontal armor at virtually all combat ranges. To defeat a Panther, M18s had to be able to hit the German tank’s sides or rear or be able to hit the lower gun mantlet and deflect a shot downward through the weaker 16mm of armor above the driver and radio-operator positions. As M4 and M18 crews realized the difficulties engaging Panthers from the front, they began to use different tactics that aimed to pin down a Panther from the front while other M4s and M18s maneuvered to try to engage from the sides. Techniques included firing white-phosphorous shells to blind enemy tanks or hitting them with high-explosive rounds. Either technique could cause inexperienced German crews to abandon their tanks.

The inability of the 76.2mm shell to penetrate the Panther’s frontal armor led to efforts to rush T4 high-velocity armor-piercing-tracer (HVAP-T) rounds (designated the M93 in February 1945) to the front. These rounds had been under development since 1943, and the HVAP design had a core of heavy and dense tungsten with an aluminum shell. The lighter HVAP-T had an improved muzzle velocity and on impact, the tungsten core would penetrate as the aluminum covering peeled away. HVAP rounds were airlifted to France in August 1944, and tests on captured Panthers revealed improved performance over the M62A1, with the HVAP round able to penetrate the Panther’s mantlet at 800–1,000yd but still with little capability against the glacis. Shortages of tungsten carbide and problems with production limited the supply of HVAP rounds and most tanks and tank destroyers only typically received two of the improved rounds each month until early 1945.

The M18’s main gun generated an excessive blast when fired, obscuring the fall of shot and leading some M18 commanders to dismount before firing and shouting directions to the gunner. The M1A1C main gun was identical to the original gun but had threads in the muzzle to allow installation of a muzzle brake to reduce the blast when firing the gun. The M1A2 main gun included the muzzle brake and had a tighter rifling twist to improve projectile stability at longer ranges. The final 700 M18s produced were fielded already fitted with the muzzle brake. Crews reported that the muzzle brake made the M18, which already had an unusual silhouette for a US armored fighting vehicle with its low profile and large road wheels, resemble a German tank even more closely, increasing the risk of “friendly fire” incidents.

The open top of the M18’s turret gave the commander excellent situational awareness, although at the price of exposure to overhead fire. The commander typically stood in the turret in action, and it was not uncommon for platoon or section leaders to dismount to guide the M18 to good firing positions. The gunner sat forward of the commander on the left side of the gun breech and had both an M4A1 periscopic sight in the top front of the turret to scan the battlefield and an M72C or M76C directsight telescope to be used when shifting to aim and fire the main gun. The turret had a rapid traverse, and gunners would swing the turret to the target’s azimuth but then use manual controls to precisely lay the gun. The loader stood in action and had access to nine rounds in a ready rack and 18 rounds in each hull sponson for a total of 45 rounds. The desire for a low profile had made the M18 much smaller than the M4 and M10, and crew conditions in the turret were cramped. The M1 gun had been offset to the right to allow the gunner to function, and the main gun itself was, uniquely, tilted at 45 degrees to allow the loader on the right of the gun enough room to be able to manipulate shells into the breech.

The M18 lacked a coaxial or bow machine gun as commonly carried on US tanks, reducing its ability to suppress enemy positions when providing infantry support. The M18 carried a .50-caliber M2 HB heavy machine gun mounted in the turret. The commander could fire the weapon against ground targets in addition to aircraft while standing in the turret – unlike some US tank designs that forced him to stand on the rear deck to use it – but he was still exposed to enemy fire. M18 crews without infantry support could be pinned in their turret by enemy small-arms fire and unable to reply, allowing German infantry to approach close enough to toss grenades into the open turret.

Technical main gun data - M18 76mm GMC

ARMOR/SURVIVABILITY
The M18 had very minimal armor, and some German antitank rounds penetrated one side of the vehicle and exited the other. The turret was a welded assembly and featured 13mm RHA on the sides and to the rear. The M1 main gun was mounted in a cast-iron plate gun shield on the front of the turret. The plate’s thickness varied from 19mm to 25mm. The hull featured welded rolled homogeneous steel plate only 13mm thick on the front, sides, and rear. The turret was open apart from a small front plate and an attached right-side plate that supported the ready rack with nine main-gun rounds. Overall, the M18 had even less armor than that carried on the M8 armored car and was barely proof against enemy small-arms fire, with the open turret vulnerable to grenades and mortar and artillery fire. Most M18 crew casualties were the result of enemy artillery fire.

MOBILITY AND RELIABILITY
After its initial teething troubles were dealt with, the M18 Hellcat had exceptional mobility and reliability. The first M18s – serial numbers 1–1350 – were powered by a Continental R-975-C1 radial engine that delivered 350hp, and gave the light, 18.8- ton vehicle its remarkable maximum speed of 55mph. After the 1,350th vehicle, M18s left the factory with the upgraded R-975-C4 engine that increased horsepower to 460hp at 2,400rpm. The M18’s high speed proved to be of less value in the field than hoped, however. Long-range movement was often compromised by the limitations of the road network and the slower speeds of other large convoys, and M18s could rarely use their maximum speed. In action, crews tended to maneuver their M18s cautiously due to the weak armor and open turret top rather than dash around the battlefield.

The M18’s high degree of reliability, however, proved a major advantage in France, with the design allowing for easy maintenance. The transmission was easily accessible and the engine was mounted on sliding rails, and with rear hatches removed could be rapidly slid back for maintenance or, if needed, full replacement.

An M18 on the move during the Battle of the Bulge. The United States failed to realize the improvements underway in German tank design and fielded the M18 in 1944 with a 76.2mm main gun that was unable to penetrate the frontal armor of the Panther. (Author's Collection)