Semi-auto...ruger 10/22. Plenty of aftermarket parts to make it as wild as you want, but you can start pretty cheap.

Bolt...for the price I really like my CZ 452 American. With a $10 trigger kit it's the best trigger I've used outside ofa $2500 Anschutz match rifle. Not to say there aren't better that I haven't tried though. Very accurate rifle as well.

For opinions from rimfire fanatics, try www.rimfirecentral.com.