...in crazy calibers. It seems that Military Gun Supply is making a new upper for the AR-15 ... in 5.7x28! And to top it off, it utilizes the P90's 50 round mags and ejects out the bottom of the old mag well.
I'm not sure, but I think it may be a simple blowback action. It states that there is no gas system to clog/malf.
If these can be confirmed to be reliable ... man. I don't know that I could pass one up.
Edit: Let's see if I can link a pic...