I don't think any automatic weapons would be desirable in a zombie outbreak. Even the ultra-light weight and accurate at close range M16 requires tons of maintenance and is difficult to fire accurately with limited training or when moving. You could downgrade to a smaller machine gun, but those are incredibly inaccurate unless you are at very close ranges. I doubt you would have to worry about the delivery not being powerful enough, I doubt you need much force to penetrate a zombie skull.
I don't know if I would go for a revolver either. I know the payload is huge but it doesn't have many rounds and I bet it requires a decent amount of maintenance. Plus it might not handle well with varying weather conditions. I'd try to find a low-maintenance pistol with a huge magazine.
Or my personal favorite, automatic shotgun with optional explosive rounds:
Re: Zombie contingency plan.
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