The 12ga FH isn't the first wildcat case built on BMG brass, it was preceded by
target rounds like the necked down 338 Talbot, the cut off 700 DKT for big game,
necked down to .416 and . 458, and more, but the following one was way
before them, in my favorite rifle caliber, for .585" bullets.

One of first modern era 585 caliber wildcats, since WW2, the 575 Miller-Greiss Magnum,
was first mentioned in the fifties, It was made from BMG brass shortened to 2.70" long
and necked up from 50 cal to .585". But actually if cut off first, it would be annealed, necked
down to hold 585 bullet. Fairly sharp shoulder.

They rebated the rim quite a ways down to .618" to maybe fit 98 Mauser bolt.
Maybe used a special hogged out, reworked action. Listed as
shooting 750gr bullet about 2400 fps. It holds about 150gr of ball powder under
a 750 gr bullet, maybe little less than 3" 50cal spotter case, where our 585HE holds
180gr under a 750gr'.

We made one for experimenting on a full length BMG case.With rim left same as the base
and would need a BMG or PH sized bolt and action. I'm glad to be a big bore wildcatter,
and happy to be in the company of big bore nuts, old and new. Years ago when guys did
big wildcats on BMGs, they took what was available, full length BMGs to rework, and but
maybe back then they could have used the 3" BMG spotter case, by shortening the neck,
if they could have found couple barrels full.

But it takes a lot of work to reform and shorten cases that much like M&G, with the multiple dies,
multiple annealings, and case trimming and extreme reworking of regular rifle actions,
which is why I like our 585 case, simple and straight, any regular, long bolt action, or
falling blocks, break actions, etc, a 585 good for dozen or more heavy loadings,
and moderate loads, last forever..

For our 585HE , made it a size, actually new belted size, to fit regular size, long bolt guns,
(not BMG actions)that didn't have to be used from tripod so to speak, and could be put in a favorite
gun of many, the Ruger #1, other falling blocks, without taking metal out of the feed trough.
To get the same and/or more powder space as a cut back, really shortened, BMG case, like M&G,
wildcatted to 585, or other short cases, we made longer case and worked with guys
with a group buy, to get a factory run made. A miracle. We thank all gun nuts for the help.

Here is picture from the great site, cartridgecollector dot net, of the M&G Mag .Ed