I would say 7x57 is the the closest thing to plug-n-play, plus barrels, reamers, and ammo are easy to come by. Another easy plug-n-play is the Sweedish 6.5x55. The 8x57 is another obvious fit but, sporter barrels are a little harder to come by.
The 7.65x53 Argentine is another catridge that feeds very well but, once again reamers and barrels are hard to find.
Longer cartridges like the 30-06 can be readily made to chamber but they do require extra work.
I've done a couple 308s and they don't feed all that great in my opinion, at least not without extra work.
I have 2 military 308 conversions, 1 Israeli lg ring and a Spanish sm ring, neither feeds worth darn.