I know Ruger and Mossberg both make .223 caliber rifles that will fit an AR15 magazine.
What I'm looking for, and haven't found yet, is a list of other calibers that will feed out of an AR15 magazine.

I've been wanting a bolt gun to use with my AR15 mags, I decided to look into what other calibers are available options before shopping for a rifle.

I'm not averse to assembling my own bolt gun, or making modifications to a factory gun in order to use an AR15 magazine.
I would like a Remington 700, I see Magpul offers stocks and magazine wells for this rifle.