Just never heard of a factory rifle that won't take factory ammo. While Cooper may be on the higher end of the spectrum, I'd still expect it to take factory fodder.
A rifle chambered for an AI cartridge will normally fire factory ammo of the parent cartridge, but a .243 AI isn't a SAAMI cartridge so there isn't "factory" ammo for that gun. That ain't Cooper's fault, it's inherent to the nature of a non SAAMI spec reamer. Having a rifle with a short throat, particularly with a non SAAMI cartridge, is not what I'd call a problem, though it might require particular ammo.
