 Firearms companies everywhere were trying to achieve a type of practical, light and cheap ammunition.Micro-caliber (less than 5mm / .20" caliber) ammunition makers wished to achieve high velocity, flat trajectory and good penetration while keeping weight and recoil low so as to facilitate controllable use in individual automatic weapons.The 4.5mm MKR was essentially a .22 WMR rimfire case necked down from 5.6mm to accept a 4.5mm bullet, for use in the Interdynamics MKR rifle and carbine.