Thanks guys.
What I was hoping for was some internal case dimensions, but it looks like I ran out of luck. I only own one center fire rifle, and that is my Ultra Varmint .223. I used to own a 6.5x55 Swede (awesome shooter), and a Marlin 30-30, don't know what I was thinking when I sold those.

I am a muzzleloader NUT and have 12 of those and several shotguns of several makes and a couple handguns. I have reloaded for a .44 Rem Mag and shotguns 20+ years ago, so I have been out of the center-fire loop for the most part for several years with the exception of the .223 which I do not load for.
What I really was looking for is a cartridge case that takes a Large Rifle Magnum Primer that can be cut down to just the last 1/2" or so for a primer carrier that can be used to ignite a powder charge in a H&R/NEF muzzleloader. I would like the internal dimensions to be very close to .500" I.D. Will the .50 AE and the .500 S&W cases accept a large rifle primer? Are the large rifle primers and large pistol primers the exact same dimensions? I have heard yes, and no.
I did find that the .300 Win Mag, and .300 RUM cases would be very close to what I am looking for, cut down of course. Would like to find something that is easy to get that don't cost an arm and a leg. Any other rifle cartridges that may have a .500 I.D. near the back half inch? I inderstand these may have an internal taper down toward the case head and that would be ok to a degree. Thanks for your help.