If you need a small package without sacrificing much thermal performance due to a shorter barrel, the 7.62×39 is perfect for a SBR AK or an AK pistol. Unlike the lighter 22 cal (.223/5.56×45) which relies on high velocity for it’s devastating thermal ballistic results. There are some premium 5.56×45 loads that work great in shorter barrels such as the popular 14″ lengths but these loads are expensive and can be hard to find.
The 7.62×39 was design for a 16″ barrel. In the image below, notice that the velocity differences in longer barrel lengths isn’t all that much.
The strengths and weaknesses of the 7.62×39 are overly debated. This is one of the strengths of the old Soviet caliber. Equipped your short barrel AK-47 with the Hornady SST or Federal Fusion and you’re golden. Those two are one of the top best loads you can find for the 7.62×39. Here is the snippet from an article:
Here are potential untapped strengths of the 7.62×39. First, because of the relatively small powder volume versus bore size, relatively little performance is lost as barrel length is reduced. This makes the 7.62×39 a perfect candidate for very short carbines. For example, a load that replicates M43 using Accurate #1680 powder will lose only about 70 fps when barrel length is reduced from 16 inches to 14 inches. Going down to a 12-inch barrel only loses another 83 fps, while a 10.5 inch barrel will still fire the 123-grain bullet at just over 2100 fps. The 7.62×39 is an ideal cartridge for a short-barreled rifle, provided a bullet that produces good terminal ballistics is used.