Considering how much damage I did back in the day with a single shot, single pump bb gun it's probably a good thing I didn't have the option of full auto.
A few years ago I picked up one of these guys though
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Umarex Steel Storm, powered by a pair of 12g CO2 cartridges. Strangely enough my 9 year old son isn't impressed with it, but every adult who tries it out wants to know where they can get one for themselves...
I saw one of those advertised a few days ago, I'd go broke buying BB's and CO2 cartridges. I suspect they'll sell a kajillion of them.