The advent of jet engines for both fighters and bombers posed new problems for interceptors. With closing speeds of 1,500 ft/s (457 m/s) or more for a head-on interception, the amount of time available for a fighter pilot to successfully target an enemy aircraft and inflict sufficient damage to bring it down was vanishingly small. Wartime experience had shown that .50 caliber (12.7 mm) machine guns were not powerful enough to reliably down a bomber, certainly not in a single volley, and heavy cannon did not have the range or rate of fire to ensure a hit. Unguided rocket weapons had been proven effective in ground-attack work during the war, and the Luftwaffe had shown that volleys of their Werfer-Granate 21 rockets, first used by elements of the Luftwaffe's JG 1 and JG 11 fighter wings on