Just thought I'd add my two bob worth on this old topic. I've been in on the hunting / culling of a few dozen camels. Here is a list of calibres that I, my family and friends have used successfully, and wouldn't hesitate to use again:

6.5x55mm, 7mm STW, .30/30, .308, .416 Rigby.

I don't recommend the .22-250 at all, although my Dad (who is an excellent rifleman) shot one camel in the head with it for a case of "lights-out".