Crazy, this is only what I have heard in the past, so take it with a grain of salt if you want.
Reportedly, the 5.45MM bullet was designed primarily for fragmentation, so against an armored combatant the weapon is less efective. However, there is a contreversy as to wether the bullet has any actual efective penetration. A test preformed on balistics gell and live pigs by an American arm's expert, Dr. Martin Facker, indicated that the 5.45, in his words was "No more effective, even at close range, than a hand-gun bullet." This comes from a comparison to damage from a 9x19MM FMJ round. However, the 5.45 would have much better armor penetration and this is under heavy contreversy.
The Tumbling Effect is bassed on something called a 'temporary stretch effect', which is exactly how it sounds. However, most organs are usualy too ridgid to be severely effected by this with the exception of the Liver (This was also the test with the pigs, but it should be noted that a Pig's internal organs are extremely similar to those of a human, down to the point where a human being can accept the organs of a pig and vice-versa) which was not as ridgid as other organs. This is also another point of controversy, as the tests results in many ways are not considered valid, attesting to the dificulty of testing Terminal Balistics with any kind of round.
Being a lighter round, the AK-74's 5.45MM ammo is likely more acurate, and some results have indicated that it's tumbling in a human body can produce effects with temporary stretch cavities that are similar to 7.62MM and Tungsten-Penetrator, non fragmentation 5.56MM rounds.
I know alot of this sounds contradictory, but testing how bullets affect a live subject is tricky to do. Personaly, I beleve the stories of the Afghan milita who coined the rounds 'The Poision Bullet' after the injuries they were given.
The round is a contreversial one, as many in the Russian defense feild debate as to the continued use of it over 7.62MM or adopting 5.56 or some other medium-strength round. In fact, from things I have heard, Russian Troops in Chechneya will often use AK-47's over the AK-74 for the missions they go on. And recently, Kalashnikov made an AK type that fires 7.62MM, but is constructed similarly to the AK-74. (This is one of the Ak-100 series.)
That's about it. Hope that helps!
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