When they installed a bomber cannon on an M1 Garand, the Japanese nicknamed them “monster weapons”.

The casing was attached to the left side of the receiver. A standard cloth ammunition belt could be inserted into it. The belt fed directly into the weapon. This allowed a single marine to carry and operate the weapon without an assistant. On December 5, the first weapon was completed. Grevich and Little took it to a secluded area of ​​the base’s firing range and loaded it with a 100-round belt.

Grevich shouldered the weapon, aimed at a target 50 meters away, squeezed the trigger, and the rifle fired. 1,200 rounds per minute. The sound was distinct, crisper than that of the M1919, and faster. The recoil was manageable thanks to the stock. The bipod ensured the weapon’s stability. Grevich fired three bursts of ten rounds. All the bullets clustered within a 45-centimeter circle.

But a problem arose. After 30 rounds, the barrel began to smoke. After 50 rounds, it was burning hot. After 75 rounds, the weapon stopped firing. The barrel had expanded from the heat. The A&M2 was designed to be cooled by a high-velocity airflow. On the ground, no cooling was possible. The barrel overheated in less than two minutes of sustained fire.

Grevich and Little considered various solutions. Adding a water-cooling system was impossible, as it would have made the weapon heavier and more complex. Similarly, modifying the barrel diameter required machine tools they didn’t have. The only solution, therefore, was to change how the Marines used the weapon. Short bursts of 5 to 10 rounds allowed the barrel to cool between each salvo.

It wasn’t ideal, but it was acceptable. By December 20, three weapons were finished. By January 5, all six were. Each weapon weighed 11.3 kg, 3.2 kg less than the M1919 A6. Their rate of fire was three times greater. The weapons were painted olive green. Grevich christened his Betty Anne.

The others received similar names. These names were stenciled in white paint onto the receivers. On January 7, 1945, the 5th Marine Division embarked on transport ships in Hawaii. The six modified weapons were packed in equipment crates. No official documentation confirms their existence. No Marine Corps armorer had inspected them. No ordnance officer had approved them.