Apparently since Ruger did so well with knocking off the Keltec 380 they should get the machines a rolling on copies of a Glock compatible Keltec Sub2000... Agh but the egos of manufacturing will never let that happen.
Despite the typical ego'esk mentality of it will only work with my guns and magazines, kudos to Keltec for making the Sub 2000 available in a couple different versions that will fit all the major manufacturers .40 or 9mm pistol magazines.
There are two things that truly make the Keltec Sub2000 unique and so popular you can't find any. First is that pistol magazine compatibiliy so that your pistol mags you are already carrying will work when used in the carbine. Need a couple extra rounds downrange but your carbine mag has run dry? No problem pull the mag from you pistol pop it in and start shredding targets out to 150 yards all without having to drop to cover to reload a carbine specific mag.
Second is that super cool folding capability. It all just works.