You can call it knock down power, you can call it stopping power, you can call it lethality, or whatever semantics you want.
At the end of the day, from my personal experience, and others that I know, 7.62x39 and 7.62x54 is more effective at incapacitating/killing than 5.56x54 rounds.
I have also seen from personal experience U.S. troops, engaged with the enemy, pick up enemy weapons for use against the enemy.
How long some french bastard blinked his eyes after he was decapitated has nothing to do with it.
Generally, soldiers are very impressed with the effectiveness of enemy weapons, as they see the effects on themselves and their buddies.
They are less impressed with the effect of their own weapons, as they often don’t see the effects on the enemy.
Thanks for your service.
5.56x45, I presume you mean. There are two major varieties, the M855 (aka S109) with 62 grain round and the M193 with 55 grain round. Both work about the same, penetrating and rotating, and often fragmenting as they turn sideways. The rounds nutate (point moves like a figure from a spirograph) and may not have much pitch or yaw when they strike meat, in which case they may penetrate (through and through) without doing much damage. If they do have a bit of pitch or yaw, that permits them to turn sideways. Nearly all bullets will turn sideways (and eventually travel back end first) if the path in flesh is long enough.
7.62x54R is quite close ballisticly to 7.62x63 aka .30/06, and it reacts much the same.
7.62x39 (123 gr @ 2250 fps) is actually close to both the .30/40 (220gr@ 2000fps) with which TR’s Rough Riders went up Kettle Hill. and the 7mm Mauser (173 gr @ 2198 fps) which opposed the US. The Mauser was probably superior due to its faster clip loading, but both rounds penetrated, rather than causing much tissue damage.
In any case, my limited understanding is that a hit with one round should never be expected to take a man down, this side of a 120mm. Audi Murphy got hit once by an 81mm mortar, and survived with minor injuries. Best practice is to use the many bullets technique of engagement.