In the Adventure Challenge I chose a 22WMR because you get a little more penetration and lethal range than with the plain 22 and still have a low weight per round compared to the center fire cartridges.
http://www.professionalsoldiers.com/...=18201&page=14
As far as 300 yard shooting goes, most people recommend the subsonic Aguila to avoid the transition from supersonic to subsonic and preserve accuracy. I have only tried the high velocity stuff and my 10-22 is not very accurate in the first place and the scope is pretty worthless for precision shooting at that distance. I would never have hit the roast with my rifle.
http://www.midwayusa.com/product/250...ead-round-nose