I recently put together a 9mm AR, and I really like the set up. But with the magazine adapter, heavy buffer, and bolt, it is a pricey item to set up. Yet a 9mm blow back carbine seems like a fairly simple item to put together. A thread caught my attention, and I did some reading on the PPSH, and noted the simplicity of the action. It got me to wondering if something similar could be accomplished with an AR 9mm. The main item of note was that the recoil spring was contained within the reciever. If an AR was set up similarly, it would open up the option of a folding stock or 9mm pistol with no stock.
Reading up on the PPSH, it was pretty clear that the Russians learned a hard lessons from the Finns, who employed the Suomi KP/-31 (a 9mm sub gun) to great effect. The Russians responded by ramping up production and use of the PPSH, which proved itself throughout the war. According to the accounts I read, the 9mm Finn sub gun was used heavily and played a pretty large role in their defense against the Russian invasion. It demonstrates the point that if your ranges are fairly short, a 9mm carbine can clean up, and having an intermediate rifle cartridge with a 500 yard range is overkill, but that's another topic.
Anyway, all of this is just an observation. A 9mm AR is probably a better system than any of the WW2 9mm sub guns. It is just somewhat surprising to me that given their history of performance and simplicity, it seems pretty tough now-a-days to get a well performing 9mm carbine.
The PPSH 41. The excellent photos are care of this thread, and I thank him for the great write up and post he did.
http://www.m4carbine.net/showthread.php?32181-PPSh-41-Submachine-Gun
Overall, you can see the fairly compact size of the carbine.
Notice the size of the bolt and action. For a blowback action, could a bolt and recoil spring fit within the reciever of a standard AR? Then you could lose the buffer tube and then fold the stock. Just a bolt and a spring, no buffer or tube.
The recoil spring is captured in the receiver, and does not extend into the stock. Note the overall simplicity of the action.
Overall view of the action.