I'm interested in 7.62x39 due to the size and power of the round in a short barreled weapon.
I suppose I could look for a 7.62x39 AR,
7.62x39 and 300 BLK are very similar rounds in terms of ballistics. Much of what was written above about performance from a short barrel is the same for the 300 BLK. A 300 BLK is after all a 7.62x35. If it were me, and I was considering AR style guns I would get a 300 BLK over a 7.62x39. The 300 BLK is better suited for use in an AR. There are lots of good 8" 300 BLK uppers. There is no real good way around the buffer tube though. Even if you could do something with a stock like the ones on the HK416C (pictured below) or ACC Honey Badger its still not as short as a folder. Personally I find an 8" 300 BLK SBR to be an overall better/more versatile package than a 7" mini draco SBR. It might not be better for a narrower set of requirements/priorities.
Also if you are open to an AR platform and power is a big priority the big bore ARs (458 socom etc) also perform rather well out of 8" barrels. While they are not long range guns (and neither is a 7" AK) they are pretty big hammers within their practical ranges.
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If going as short as you can possibly get when everything is folded/collapsed/etc is you first priority it will be tough to get shorter than 7" mini draco. Most mini draco SBRs, IIRC, tend to be about 17" or so with the stock folded (the exact length obviously depends on the exact folder used) and around 25" with it extended. Depending on the muzzle device used you can add a couple inches as well. My experience is the mini needs a pretty good flash hider if you intend to fire it more than once in the dark. In my testing, even with an A2 birdcage style hider, the mini gave off a seriously blinding fireball when fired at night. The same hider made regular draco manageable. I'd like to try a vortex on a mini but haven't had a chance. It would add more length.
Depending on your exact uses, something like a Kel Tec sub 2000 might be of interest. It is really short when folded. Folded the sub 2K is 16" long. It is 29.5 when it is open. It has some drawbacks It of course has KT build quality, cannot fire when folded, and is chambered only in 9x19 and 40 S&W. That said a 9x19 based on my experience picks up speed in a 16" barrel. With a hotter load it can push a 124 grain HP to right around 1500 FPS. Perhaps one could get more if they used hotter loads still or something like Winchester 127 grain +p+ loads. I've never tried to really hot rod it and see. I've seen people report 1650 with 135 grain 40S&W out of the sub 2K. That isn't to far off the muzzle velocity of a 123 grain bullet getting the 1800-1900 FPS that a lot of people report. Now again certain uses might require us to look harder at ballistic coefficients, bullet construction, etc. Velocity and energy numbers only tell part of the story. What are the realistic distances to be shot and what is to be shot at those distances?
The Sub 2000 has some notable advantages. It is cheap. It takes mags for various popular pistols. MY preference is the glock versions. This allows use of 33 round mags and 50 round drums (for 9x19 models). Generally ammo has been cheaper. It isn't an SBR, so no stamp or other requirements that go along with an SBR.
One other non-AK gun worth looking at might be a Rob arms XCR mini. It is 18" OAL when folded and (at least in theory) can be had in 5.56, 7.62x39, or 6.8 SPC. Why no 300 BLK? I don't know. The 6.8 SPC does well from that barrel length though. There is much that could be said about the XCR but its not worth mentioning unless it is something that is even in the running to begin with.
