The .224 BOZ was developed by Civil Defense Supply, located in the United Kingdom, and was begun as a Special Forces ammunition project for police and military use. In short, the concept was to design a new pistol cartridge that would deliver significantly more power than the current 9mm Parabellum as used in both handguns and submachine guns. The two main requirements were that the cartridge be adaptable to existing firearms and that it be able to penetrate CRISAT armor (1.5mm titanium plate reinforced with 20 layers of Kevlar) and generate a PIH level of 0.6 or greater.
It was decided that the bullet chosen be one of proven design and so the 5.56mm round as used by NATO forces was selected. Not only was there a significant amount of data already collected on this bullet design, but it was also available in numerous configurations such as armor piercing, tracer, ball, and frangible. In looking for a starting point for the cartridge case both the 9mm Parabellum and the .40 S&W were looked at and then quickly dismissed as possible candidates because both lacked the case capacity to generate the required energy and velocity. The 10mm Auto was then chosen both for its power potential and existing firearm platforms readily convertible to the new cartridge.
The original test-bed for the .224 BOZ was a Colt Delta Elite. Using lightweight 50gn 5.56mm bullets velocities ranging from 1,750fps to 2,200fps were obtained. Eventually the Glock 20 was chosen as the foundation for the .224 BOZ PPW (Personal Protection Weapon) due to its "positive lock between barrel and frame." (It must be noted though that in this caliber a 5.5" barrel is considered "compact" and a 6.5" barrel is "standard.") For the carbine/submachine gun platform the HK MP5 was selected Not only is the MP5 series of submachine guns one of the most popular around the world, but it is also available in 10mm (though no longer in production). Royal Ordnance and Heckler & Koch worked together to develop the .224 BOZ PDW (Personal Defense Weapon) which is basically an HK MP5/10 chambered for the .224 BOZ round. Reportedly there is also a .224 BOZ PDW based on the AR15 platform, but apparently this is still under development.
Sounds like a waste of time to me. Use a 5.56 caliber billet in a necked down pistol case in a rifle (AR-15) that already used the caliber. A 5.56mm round is effective for mere feet out of a short barreled rifle 14.5 inches or less. While it may go through the armor, its' effectiveness on the person would be very minimal in the MP5 series.