5.56 as mentioned.
Why is surplus not an option?
5.45-39 is cheap now and has a lot going for it.
To get more power you would have to go up to .308 or maybe 30-30. Readily available and not painful.
.22 Mag is a gain over the .22 but it not cheap in my mind. About the same price as 9mm. There are some 9mm carbine options worth a look (Beretta Storm) or you could get a .40 version of that for more power (but more $$ ammo).
He who dares wins.
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