Micro-caliber (less than 5 mm (0.20 in) in diameter) ammunition makers wished to achieve higher velocities, flatter trajectories, and greater penetration while keeping weight and recoil low as possible to facilitate controllable use in individual automatic weapons.

Due to the combination of a thin-walled rimfire case with a high-pressure powder charge, the cartridge needed good external support so the MKR rifle was designed to use locked breech, gas-operated action with a vertically tilting bolt rather than straight blow back operation.

The rifle and carbine, which differed only in barrel length, were fired from the open bolt position to prevent cook-off ignition of the thin walled cartridges.

[1] The 4.5×26 MKR was claimed to be on par with a 5.56×45mm NATO M-193 bullet at up to 300 m (980 ft), while one could carry 3.5 times as many of the lighter weight cartridges.

It was potentially lethal at up to 750 m (2,460 ft) with 80 J (59 ft⋅lbf) and its tendency to tumble on impact was twice that of the 5.56 M-193 bullet, thus ensuring more effective transfer of its diminutive energy to its target, especially as its tip tended to bend.