ammo performance
Those you can easily look up - Google will tell you a bunch of reliable sites which come up with congruent info.
effective range
A target is in effective range, if you can actually expect the weapon to have effect reliably (not random hits only).
For the AK-47 it's just the case that @400m you'll have to point it a little bit much "at the sky" rather than at the target, and the slow bullet is prone to wind drift etc. or to miss just because the target moved away. So you simply can't hit a target reliably at that range. Therefore it is generally attributed with an effective range of 300m.
The AK-74 is attributed with an effective range of 400m because of the faster bullet. It is also said that the small projectile has low stopping power, as it seems to simply go right through without breaking up (as the soft-core fullme tal jacket NATO 5,56 from M16 does). This would result in a lower HEkill factor, gamewise !?
The MG-43 @ 3000m is a little bit different, as this surely isn't aimed, but plugging area fire. Using a heavy tripod, the MG is "converted" into a HMG. Traverse in every direction can be limited seperatly, therefore a "kill zone" can be set up beforehand and later "sprayed" with bullets.
Direct fire using the tripod and attached scope is possible up to 1600m or something, IIRC from the time I had to fiddle around with it's successor MG-3
