The 4.5mm MKR was originally developed by Interdynamic AB, a Swedish company, in the 1980s, right around the time of the "micro-caliber" craze, when firearms companies everywhere were trying to achieve a type of practical and cheap ammunition. Micro-caliber (less than 5 millimeters) ammunition wished to achieve high velocity, flat trajectory and good penetration in automatic weapon, and with low recoil, but the tiny size of the bullet required a simple blowback operation style rather than more complex forms of operation. Lightweight bullet,1.58 grams, achieved a muzzle velocity of about 3270 feet per second. The effectiveness of this bullet was claimed to be on par with 5.56x45mm NATO bullet up to 300 meters.
Due to concerns about stopping power, reliability and problems with using rimmed rimfire ammunition the MKR project was abandoned at the prototype stage and thus the 4.5mm MKR never saw service or any kind of widespread use.
The 4.5mm MKR was essentially a .22 WMR necked down to a 4.5mm bullet, designed to be used in the Interdynamics MKR rifle. The Interdynamics cartridge featured curved taper and a 4.5mm caliber pointed bullet, made of solid brass.