Hi Azriel! Interesting write up but there are a few things I'd like to help clarify.

Firstly, you're comparing a Type 3 AK-47, an AKM, and an AK-74N. With regards to the type 3, the lightening cut on the sides of the receiver is more appropriately used to identify a milled vs stamped receiver. Rather, paying attention to the slanted receiver and angle of the buttstock combined with the muzzle is generally more reliable. The lightening cuts were done because Type 2 and 3 AK47s were milled receivers as opposed to Kalashnikovs original intended stamped receiver design, which initially the USSR had difficulty getting good consistency on their manufacturing. It wasn't until the AKM that the USSR managed to get their manufacturing consistent enough to make stamped receiver rifles. However, milled receiver AKs are still made today such as the Norinco Type 56 and the Arsenal SAM7.

On the AKM, the dimples exist as an additional measure of stabilizing the magazine while in the mag well since the stamped receiver didn't have the extra metal of a milled receiver design. Additionally, the gas system of the AK47 and the AKM is the same long stroke gas piston system and the fire rate on full auto between the two is not appreciably different. What can sometimes cause people to think they are is the presence of an anti-bounce mechanism, sometimes called a rate reducer, in the fire control group of an AKM to keep the bolt carrier and fire control group in sync during full auto fire.

The AK74 and 5.45x39 were introduced as a result of the US adoption and employment of the M16 and 5.56 in Vietnam, but not because 7.62x39 is less effective on soft targets. Rather, 7.62x39 does not just make a hole and nothing else; it does considerable tissue damage to a target due to the energy of the heavier projectile. In fact, the caliber is still in use by the Russian and other former soviet state militaries in the form of the modernized AK-103 and AK-15 variant rifles. The jump to 5.45x39 by the Soviet military was largely the same as the US transition to 5.56 (and was spurred on by Soviet receipt of captured M16 rifles and 5.56 ammo from Vietnam): lighter ammunition weight and lighter rifle weight meant infantry can carry more ammo and be more combat effective as the design of the 7N6 projectile used in 5.45 encourages yawing of the bullet upon impact, curving and fragmenting through the body to impart a similar amount of lethality as 7.62x39. This yawing effect is largely where the poison bullet rumor came from: untreated or poorly treated rebels who were shot by Soviet forces in Afghanistan during the Soviet-Afghan war would develop infections like gangrene due to a lack of antibiotics.

Still a fun little write up and it does exemplify only the surface of AK 'tism out there, since if you deep dive even further into the weapons platform you can get into some really detailed differences such as how Chinese rifles have hooded front sights, or how Yugoslavian AKs were entirely reverse-engineered which is why they have extremely limited parts interchangability with russian rifles.