The 4.2 cm Pak 41 was a light anti-tank gun issued to German airborne units in World War II. This gun was externally similar to the 3.7 cm Pak 36, and used a modified version of the latter's carriage, but used the squeeze bore principle (in German called Gerlich after Hermann Gerlich who developed the principle in the 1920s, reportedly for a hunting rifle) to boost its velocity, and hence armor-piercing ability. The bore had a diameter of 42 mm at the chamber, but tapered down to 28 mm at the muzzle. Production was terminated in May 1942 after the delivery of 136 guns. By November 1944, 41 remained in service.
Gander, Terry and Chamberlain, Peter. Weapons of the Third Reich: An Encyclopedic Survey of All Small Arms, Artillery and Special Weapons of the German Land Forces 1939-1945. New York: Doubleday, 1979 ISBN 0-385-15090-3
Hogg, Ian V. German Artillery of World War Two. 2nd corrected edition. Mechanicsville, PA: Stackpole Books, 1997 ISBN 1-85367-480-X