Because the 7.62x39 commonly has a 1-10.5" .310" 312" bore not a .308" and wont shoot the long 240gr SMK.
Shooting 123-125gr bullets they are much the same.
Before I purchased my first .300 Whisper all those years ago I actually built a subsonic AK47 by using the military surplus 311" 180gr yellow and silver tip steel core loaded for the 7.62x54, they are a very long projectile so tumbled after impact like the 240gr smk inthe whisper, they are almost as long as the 240gr but lighter due to the steel armour piercing core. Loaded in the 7.62x39 case with 11.3 gr of 2207 cycled them through the AK action and spat them out at 1050fps.
They were great for popping goats quietly when out chasing deer and also took the od rabbit and possum at base camp :thumbsup:
https://preview.ibb.co/etk05U/subs.jpg
