If you adhere to minimum case taper, put in a 23+ degree shoulder, and keep the wildcat case down to about 2.5 inches long, you can squeeze out a .458 Mag. In the full length Ruger case, trimmed to 2.5 inches, its been said that you can duplicate 458 Lott power in a standard length action with a magnum bolt face.
Many, many moons ago, the Peterson Rifle Ranch did a series of these necked up 284's. They thought the 40-284 was about the best answer, back then. I have a new wildcatted 300 RCM case, with a severe taper, which is the same taper as the 458 American. If I expanded my wildcat case's neck, I think I could still have barely enough shoulder at the .429" bore. I think this would shrink the case length from 53mm to about 50mm.
Without me ironing in my severe body taper, a .458 x 1.9 inch, 300 RCM, should be doable. It would be something like a 45 x 30-06 Ack. Imp.+, which would head space on just enough shoulder.
I don't care for the rebated rims of the 284 Win. and the new WSM's. All in all, I feel that the new RCM brass is more efficient. But I know the WSM case is the largest in capacity of these three or four, short rimless magnums.
But 75% of the 458 Lott's poop, is still a heap. Weight of your rifle, may overshadow short action ballistics shooting one of these howitzers. It only makes sense in a bolt action repeater. Using a Ruger or Winchester falling block single shot, begs for the 458 Lott, itself.
I have an earlier wildcat called the 44 Marsh, and in a light win. 94 carbine, this 44 x 356BB has about all the recoil I can appreciate, shooting it over the bench. I calculated it as a 200 yd. Elk rifle. Using the larger RCM parent case, with my existing body taper, would give me a bolt action, rimless 444 Marlin, which would still have just enough shoulder to head space on.
The closest one to this is the belted 240 Weatherby Straight, at .411". So all I say is; "be careful what you wish for". I won't do another 44 Marsh in a round barreled Win. carbine. And if you think there will be a good enough supply of 450 Marlin ammo and brass, in your future, why its game over, right there.
But for me, a 44 cal. RCM would let me use the 400 gr. 444 Marlin cast bullets, being sold by the owner of this site, Bear Tooth Bullets. Plus I could use the tough Freedom Arms 44 mag. jacketed stuff which will hold together in big game, at these velocities. I would expect about 2400 fps., out of a Hornady, 44 cal., 265 gr. Gummie Tip bullet.
You might get some really high velocities out of the light weight 45 cal. Barnes homogenous metal bullets, which the Rifle Ranch folks never had any chance to play with, back in the day.
If I wasn't already so invested in 44 cal. equipment, I might look towards a 404 RCM short magnum, in a short barreled, CRF bolt action rifle. My severe case taper, in both long and short Ruger parent cases, does give better feeding out of my M-98 Mausers. These Cohibas, work better with round nosed bullets, and that is a strike against the 44 cal. bullets made for tubular magazines. You will also run into this problem with many of the lighter 458 jacketed bullets, made for the Marlin 1895's.
I just barely creep up into the working range of the 265 gr. Gummie Tips,(1830fps), with my 20 inch barreled 44 Marsh carbine. I kind of doubt that you will get 500 gr. bullets made for a 458 Win. Mag. to work right, out of any 284 win. parent case. But even this wildcat has been done before. It was really right out on the edge, or so the Rifle Ranch people said.
