The Littlejohn adaptor was a device that could be added to the British QF 2 pounder (40 mm) anti-tank gun. It was used to extend the service life of the 2-pounder during the Second World War by converting it to squeeze bore operation. "Littlejohn" came from the literal anglicization of the name of František Janeček, the Czech designer and factory owner who had been working on the squeeze-bore principle in the 1930s and his son František Karel Janeček had brought his know-how to Britain after he fled from the Nazi-occupied Czechoslovakia.
The adaptor took the form of a reducing bore that was screwed onto the end of the gun. This was coupled with a round formed from a hard core (tungsten) inside a softer metal casing - the armour-piercing, composite non-rigid (APCNR) design. The round upon firing travelled the first part of the bore as normal. On entering the tapering portion the softer and malleable metal of the outer shell of the round was compressed - from 40 mm to around 30 mm. The round when it emerged from the adaptor now had a smaller cross-section compared to its weight. Together with the higher pressure developed in the barrel of diminishing internal volume compared to standard cylindrical bore, the APCNR round, called APSV (from armour-piercing super velocity), travelled faster, over a flatter trajectory. The Littlejohn adaptor/APCNR combination gave the 2 pounder a similar enhancing effect as the APDS round gave to the larger QF 6 pounder gun.
The muzzle velocity of the APSV Mark II shell was 1,143 m/s compared to the 792 m/s of the normal 1.2 kg APCBC shell. The lighter Mark I APSV shell was capable of penetrating 88 mm of armour at 450 m at a 30 degree angle of impact.
The adaptor was chiefly used on British armoured cars e.g. the Daimler which had been designed and built earlier in the war and could not be readily fitted with a larger gun. As an adaptor to the existing gun it could be removed at any time so that normal rounds could be fired.
- 2.8 cm sPzB 41 - German anti-tank gun working on the squeeze bore principle.
- 4.2 cm Pak 41 - Most comparable German anti-tank gun working on the squeeze bore principle.
- 7.5 cm PaK 41 - Another German anti-tank gun working on the squeeze bore principle.
- Zaloga, Steven J., Brian Delf - US Anti-tank Artillery 1941-45 (2005) Osprey Publishing, ISBN 1-84176-690-9.