I thought he was wanting a medium sized caliber to use in an AR15, my bad. If that is not the case then the 243, 25-06, 260, 308 or MOST anything really would do the job.
If I were to choose a bolt gun it would be my 25-06 hands down, that thing just flat kills stuff dead. If I were to use one of my AR's then it would be the 243WSSM it also does a whicked job of putting down wolf sized animals.
It sounded to me as if he was looking for alternative caliber to his 223 built on an AR15. There are several caliber that would do nicely and several have factory ammo available. 6.5 Grendel, 6.8SPC, 243WSSM, 25WSSM.