Well...howbout an AR57?
An AR57 commonly refers to a kit gun or a factory installed conversion to the AR15 style rifle that converts it to shoot the FN 5.7x28 ammo used in the FN P90 submachinegun and the Five-Seven handgun. The gun transforms into a top-loading style weapon that takes factory P90 magazines (10,20, 50 rounds). The ballistics are actually pretty similar to the .223 but the recoil is quite a bit lessened due to the shorter shell. The original ss190 5.7 rounds are armor piercing, but I don't think you can purchase those. I'll stick to the ss195 which shoot about 2,500fps.
The AR57 upper can be configured with 6, 11, or 16 inch barrels and are threaded at the tips for silencers and flash hiders all. They only come with quad railing, which is cool, and all other AR15 parts work fine since the lower is just a normal AR15 lower.
Some people have been converting M4s into these things to make basically a P90 with AR ergonomics.
Anyway, I plan to buy a stripped or complete lower end by DPMS or Bushmaster (forged) and assemble the thing myself and probably selling the old parts away. I'm basically looking at a lot of Magpul parts (the people that brought you 90% of the new AR stocks/forarms/magazines you see now, also the Bushmaster ACR was actually developed by Magpul).
Again, this is a gradual project and it will most likely take me all summer to save up and buy the pieces and assemble them, AR15s are notoriously easy to assemble, but I may buy a manual/book on the subject just in case.
Magpul stock and other parts
Magpul MIAD grip system, totally customizable, includes an insert to hold 3 emergency rounds/a spare bolt and firing pin/or spare batteries for your tactical flashlights
An aftermarket part for the aftermaket magpul grip is the Samson Survivor kit, which is actually a full cleanup/tuning kit for an AR15 that clips into the grip just like a pistol magazine.
