 Princess Royal received two 4 @-@ inch Mark VII guns on HA MkII mounts capable of 60 ° of elevation in April 1917 and a pair of single 2 @-@ pdr MK II " pom @-@ poms " were added in April 1919 . They fired 40 @-@ millimetre ( 1 @.@ 6 in ) shells weighing 2 pounds ( 0 @.@ 9 kg ) at a muzzle velocity of 1 @,@ 920 ft / s ( 590 m / s ) to a maximum effective range of 1 @,@ 200 yards ( 1 @,@ 097 m ) . Their practical rate of fire was between 50 and 75 rounds per minute . 
