7.62x39 is actually very close to 30-30 Winchester as far as ballistics go with the same bullet. And, if your purchasing your ammo vs reloading it, 7.62x39 is cheaper to shoot than .300 blackout. However, for shooting cheap with an AR type platform, 9mm is the way to go. But, for the 9mm AR, 100 yards is about the maximum practical range and 200 yards takes some practice. And, 9mm will shoot subsonic with 147 grain loads.
Is the 9mm you mention configured the same as pistol 9mm?