10 . 5 cm K ( gp . Sfl . ) The 10 . 5 cm K " gepanzerte Selbstfahrlafette " ( " 10 . 5 gun on armored self - propelled mount " ) was a prototype self - propelled gun used by Nazi Germany during World War II . Although it was originally designed as a " Schartenbrecher " ( " bunker buster " ) for use against the French Maginot Line defences following the defeat of France it was used as a tank destroyer on the Eastern Front . Development . It was intended to be used against bunkers at ranges beyond which the bunker could return fire . Development by Krupp began in 1939 . With the conquest of France complete the design was without a purpose so it was suggested that it be used as a heavy tank destroyer . Two prototypes were ordered , completed in January 1941 and demonstrated in front of Hitler on 31 March 1941 . If troop trials were successful it was estimated that series production could begin in the Spring of 1942 . Description . The 10 . 5 cm K ( gp . Sfl . ) was built on a heavily modified " Panzerkampfwagen " IV " Ausf " . E chassis with the turret removed and an open - topped superstructure added to house the main gun . The forward glacis plate was of face - hardened armour at 15° from the vertical while the sides were thick . The vehicle ' s most curious feature were armored compartments provided in the rear of the vehicle to protect the loaders against attack from aircraft . Similarly the ammunition bins holding 26 rounds for the main gun had thin armored covers on top . Another oddity was the fake driver ' s compartment on the right side of the vehicle that matched the real one projecting forward from the front superstructure on the left . Although the main gun was intended to engage enemy armoured fighting vehicles it could only traverse 8° to the left and right , depress 15° and elevate 10° . A muzzle brake was fitted to reduce recoil forces and a travel lock was fitted on the front deck to secure the gun during movement . For self - defense the crew carried three 9 mm machine pistols with 576 rounds of ammunition . A " Selbstfahrlafetten - Zielfernrohr " ( Sfl . Z . F . ) 1 sight was used by the gunner , a binocular " Turmspähfernrohr " ( T . S . F . ) periscope was fitted for use by the commander , and each loader was provided with a binocular " Scherenfernrohr " on a pivoting arm . Throughout most of its development it was known as " 10 cm K . ( Pz . Sfl . IVa ) " , but was redesignated " 10 . 5 cm K ( gp . Sfl . ) " on 13 August 1941 , and was also colloquially known as Dicker Max ( " Thick " or " Fat " Max ) . In the prototype vehicles the V - 12 Maybach HL120 engine of the Panzer IV was replaced by a lighter V - 6 Maybach HL66P engine . Production vehicles probably would have used the suspension and running gear of the " Panzerkampfwagen " III as it offered increased maneuverability due to a shorter length of track in contact with the ground , less rolling resistance , softer springs and greater deflection of the suspension . Combat history . For combat trials the two prototypes were assigned to " Panzerjäger Abteilung " ( " Anti - tank Battalion " ) 521 during the invasion of the Soviet Union . One caught fire by accident and was completely destroyed by the detonation of its ammunition , but the other fought successfully until the end of 1941 . It was rebuilt by Krupp during the first half of 1942 and returned to the 521st in time to participate in Case Blue , the 1942 summer offensive in Russia . It was not reported operational in reports made by the battalion in November — December 1942 . A report of 26 July 1941 commented on the vehicle : " This Sfl . ( self - propelled vehicle ) is not sufficiently maneuverable for employment in a Vorausabteilung ( lead unit ) . The limited traverse makes it necessary to turn the entire vehicle to aim at targets . This takes considerable time when done repeatedly , especially off - road because of the heavy vehicle and weak engine . In addition , because of its armor layout - only thick in the front and 50 mm lower in the back - it was built for frontal use . The vehicle can be shot into from the side and rear . Employment in a Vorausabteilung requires the ability to quickly engage targets in every direction . [ . . . ] The gun has proven itself capable of supporting an infantry attack from an open firing position by direct fire as in the Szczarar crossing . It is not possible to observe our own shots because of large dust clouds raised in front of the gun . The Sfl . must alternatively observe each other ' s fire or an observation post must be established off to one side , manned by a Richtkreis - Uffiz . ( gunner ) familiar with the crew . Because of its size , lack of mobility , and large dust cloud raised when firing , in the future the Sfl . will only shoot " Sprenggranaten " ( HE ) by indirect fire . [ . . . ] Up to now , the Sfl . has been employed in its specialized tasks - engaging concrete bunkers by direct fire and engaging heavy tanks in coordination with other Panzerabwehrwaffen ( anti - tank weapons ) . Its high penetration ability appears to be suitable for this . [ . . . ] No specific problems have occurred with the engine or transmission . The steering brakes are overstressed . Bolts in the steering slide of one steering brake have torn out three times and the brake bands have had to be changed twice because the rivets were over - heated . "