MCA Sports in Anchorage, Alaska makes a wide variety of chamber adapters, or so says their website:
http://www.mcace.com/adapters.htm
If you're itching to shoot .30 Luger ammo through your .308, apparently they can fix you up. Also .32 ACP, .30 Carbine and a bunch of others.

If anyone here has actually ordered anything from this company, I'd like to hear about it.