Rumor has it he was shot with an HK416, an assault rifle chambered in .223
You were very close...
I'm getting some good indications that it was an HK MP7 A1
that fired the Osama kill shots.
Smaller than a conventional submachine gun, the 4.6 mm MP7 A1 is a compact and lightweight Personal Defence Weapon that can be carried like a handgun yet is capable of rifle-like effectiveness.
The HK-developed 4.6 mm x 30 ammunition provides the penetration approaching that of an assault rifle round and is able to defeat the types of body armor frequently found in the hands of terrorists and criminal gangs, in particular that of the former Soviet Bloc special forces, now the standard NATO test target (CRISAT).
In response to the NATO PDW requirement document D29, HK developed a Personal Defense Weapon system that will penetrate the CRISAT vest, comprised of 1.6 mm titanium plates and 20 layers of Kevlar, out to 200 meters and beyond.
Adopted and fielded by several of the world’s leading special operations units, the gas-operated MP7 A1 fires from the closed bolt position, using a rotary locking bolt system similar to that of the HK G36 assault rifle to contain the powerful 4.6 mm cartridge.
The MP7 A1 handles like a pistol yet allows targets to be engaged like a rifle. With its 7-inch barrel, the HK MP7 A1 is capable of firing 10-shot semiautomatic groups at 45 meters of less than 2 inches. Unlike competing designs, the conventional and fully ambidextrous design of the MP7 A1 ensures immediate acceptance by all users.
The MP7 A1 is manufactured predominantly of carbon fiber reinforced polymer with embedded metal components where needed. The weapon can be field stripped in seconds without tools and requires little cleaning due to its unique gas system.