The degree of protection offered by a pillbox varied considerably: the thickness of the walls and roof generally varied from just 12 in to 3 ft 6 in (0.3 to 1.1m) or more although the commercially produced designs were often much thinner. In March 1940, General Brooke carried out penetration trials and recorded that a anti-tank gun could easily penetrate up to of reinforced concrete.Alanbrooke, War Diaries, Entry 22 March 1940. Despite such results, the thick-walled pillboxes were designated as shell-proof, whereas the thinner-walled pillboxes were designated as bulletproof.
