I agree that short action calibers are in order. I don't think you will get a complete burn on your long action caliber's powder with such a short barrel. So you wasting powder, recoil, and muzzle flash for no reason.

One thing to consider, though it may not be an issue, is that if you intend to use the gun in areas where rifles are not allowed, like a shotgun/handgun/muzzleloader only type of area common in more urban environments, most Department of Natural Resources (or whatever it is where you are) classify handgun rounds as straight walled cartridges, thus the rifle cartridges would NOT be legal there. You might look at some of the larger pistol calibers like .454 casull and such. It is just a thought and may not apply to what you want, I just want to make you aware of some of the laws.