You're ignoring two factors...controllability and portability. The 12 gauge blows a lot of them lead *****, but at the added cost of slow recovery from the recoil. Also, buckshot is a round ball, and that shape gives terminal ballistic behavior that tends to follow the contour of the impact point. Unless the zombie you're shooting is weakened or decayed, cranial penetration by #3/#4 buckshot is marginal at best. And both the 12 and the AK require two hands to carry comfortably. So, you were outside in ZombieLand for...what? Food? My, won't you be popular when you report back to your survival clan with half your ammo gone, no food, and ten thousand curious Zippies following you to your secret hideout 'cause you made so much noise.

Miz Roo has just brought up the point that both high volume (buckshot) and high velocity (7.62 x 39) suffer from the unfortunate tendency to backsplatter as much as 10-15 yards. If you shoot with your eyes (or worse, mouth) open, microdroplets can infiltrate your respiratory or digestive systems and before you know what happened, no amount of Right Guard Winter Blast deodorant will suppress that air of distinction that will mark you for special treatment by your peers.

I know this all sounds so hopeless, but Z-sports don't have to be dangerous. One of my favorite pastimes is to get out on the mall roof with a bunch of bricks. We call it "Bombing Zombies". It's a lot harder than it looks!