Extra mags can be expensive, if you get the mossberg AR mock ups. If using standard Mossberg magazines, they can be a pain to manipulate.
Personally, I'd go with S&W (read up on current issues with them) or a dedicated 22LR AR upper.
The Colt branded Walther M4 Clones are ok, but not as accessory friendly a S&W, and again, expensive mags. They do have an A2 clone, though.
If you go Mossberg, I ma know someone selling a used one.
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