The Thompson Autorifle

🔒 The Blish Lock System

The operational principle of the Thompson Autorifle is rooted in an interrupted screw delayed blowback system, ingeniously regulated by the Blish Lock. This mechanism employs a bolt featuring 85-degree angled interrupted rear locking lugs. These lugs must overcome a 110-degree rotation to disengage from their receiver counterparts. This rotational delay, combined with the angled surfaces, ensures that the bolt remains locked until the expanding gases within the barrel have reduced to a safe pressure level, facilitating the controlled ejection of spent cartridge casings.

🔫 Firing and Feeding

The bolt is designed to cock the striker upon opening, and the weapon fires from a closed bolt position. The rifle's receiver is cylindrical, housing the safety switch and rear sight assembly at the rear. Ammunition was supplied via a detachable box magazine, initially holding 5 rounds, with later prototypes adapted to use the 20-round magazines common to the M1918 BAR. The system required lubricated ammunition for optimal functioning.

🇬🇧 The BSA Autorifle

The lineage of the Thompson Autorifle extends to international collaborations, notably the BSA Autorifle. Developed by Birmingham Small Arms Co. (BSA) under license from Auto-Ordnance, this British variant was offered for trials by the British Army during the 1920s. A key enhancement was its dual-mode capability: the mechanism could be disconnected by manipulating two curved 'shutters' around the bolt head, allowing for manual operation. This feature was a specific requirement for self-loading rifles submitted for British trials during that era.

🔬 Trial Performance and Development

The BSA Autorifle participated in trials in 1927, competing against other designs, including a Colt-made Thompson and John Garand's rifle. While reports indicated it performed competently, narrowly surpassing some competitors, it did not progress beyond the prototype stage, as no design was deemed fully acceptable. Surviving examples suggest BSA produced a limited number of 'heavy rifle' variants based on the 1923 model, featuring folding bipods and finned barrels. These included both Rifle and Light Machine Gun configurations. An 'improved' variant, featuring a shorter operation cycle and a straight-pull cocking handle, was developed in 1926.

⚙️ BSA Autorifle Mechanism

The BSA Autorifle itself operated on a delayed blowback principle, chambered for the .303 British round, and fed from a 10-round box magazine. Its bolt employed an 85-degree angled thread system, delayed by a 90-degree twist, ensuring safe ejection pressures. The bolt cocked the striker on opening, and the rifle fired from a closed position. The receiver was cylindrical, with the safety switch and rear sight located at the rear.

📜 References