Well, all you have to do is get one headshot. .22's used to be used my the mob for executions. the round has enough power to enter the skull, but not to get out, so they can ricochet around inside the skull, shredding a zombie's grey matter.
Depends on the round. Remember .22 is only the caliber of the round, not an indicator of the "power" of the round which is determined by (laymans terms) the size of the casing and poweder in it. Remember the M16 5.56mm is a .223 caliber round and it will punch through the human skull, not to mention the brain buckets (helmets) issued while I was active duty. The helmet at the time was rated for slow moving rounds and shrapnel/flak, not high velocity rounds.
The .22 of mob style behind the ear executions was a low power derringer sized round. The .22 long rifle round, fired from both rifles and pistols can punch through both sides of the skull, the .22 short may not but its also designed for very up close use.
To those thinking of the AA12, or even the Daewoo made USAS12, (better weapon from my experience though both suffer issues that turn them off in my mind) the fire power idea is nice but your forgetting the issue of weight. Both weigh around 15 lbs alone without ammo. A fully loaded M16 weighs around 8 lbs (with a loaded magazine), and I can tell you that after 15 mile hump those 8 lbs even slung over a shoulder feel more than double or triple. Not to mention the amount of ammo you would need to carry

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There is a reason the Marine Corps instructs the rifle on semi-auto only and not three round burst. Conserve your ammo and make each shot count. Full auto and burst fire is a surpressive tactic first unless you are setup and dug in with clearly defined lanes of fire and can control the approach of the enemy. Once that adrenaline kicks in and you are tweaking its only a matter of seconds before you've emptied that magazine, and if you the kind of person that shakes when the adrenaline is flowing reloading becomes a much tougher proposition.
If you want shotgun I would recommend the Franchi SPAS-12. With the right load you get semi-auto goodness, along with 12 gauge slugs and terrific bracing system that once you are use to it can let you fire the thing in semi-auto mode with one hand.
Aaron
**edit wow Im taking this was too seriously lol but its fun.