.300 AAC Blackout

The .300 AAC Blackout (also known as 7.62×35mm) is a rifle cartridge developed in the United States by Advanced Armament Corporation (AAC) for use in the M4 carbine. Its purpose is to achieve ballistics similar to the 7.62×39mm Russian cartridge in an AR-15 while using standard AR-15 magazines at their normal capacity. The .300 AAC Blackout uses the same lower receiver, magazines, and accessories as the standard AR-15 rifle; you need only swap out the barrel for one chambered in .300 AAC Blackout to convert your AR-15 from 5.56 NATO to .300 AAC Blackout. 

For those who want the power of the Russian 7.62x39mm Russian (AK-47) cartridge, but like the AR-15 style rifle, the .300 AAC Blackout is the way to go. The shooting results shown here, demonstrate that there is very little difference between the 7.62x35mm cartridge and the 7.62x39mm cartridge. 

So, the question comes down to whether you prefer the AR or the AK style of rifle. And, of course, whether .300 AAC Blackout ammunition is available to you.