I'm wanting to put a new .308 barrel on an older Stevens 200 .223. The one part I cannot find is the internal stagger feed box magazine for .308. Can anyone tell me where I can find one? This is the type mag that is attached to the receiver when pulling the barreled action. Any help appreciated as I'm can't continue without one. I've tried Numrichs, Brownells, MidwayUSA, etc.