I've always wanted to get the max perfomance possible from a 300WinMag, with 180gr bullets, that's it, without exceeding more than 6000 or 7000 psi, from the SAAMI baseline at 62000 psi.

Another consideration, is not going to a mangnum-action, basically mi idea is to use the Ruger M77MkII action.

The .300 Dakota seemed a good candidate, but the basic case, is not easily available, or at least cheap, whatsoever.

Then I thought of shorten a 300 RUM case from 2.850" to 2.620" (.300 WinMag max case length), and that simple conversion should make a nice performer in a standard length action. In fact, it'd be a lengthened 300 Dakota or a shortened 300 RUM, your choose!

Please, any knowledge of such a cartridge or any comments on developing this idea? Any idea about possible perfomance?

Tks in advance!

Well, I think you can pretty much assume Dakota performance, and a little better with the lighter bullets, 165g and under. But in case you aren't aware, the Dakota is based on the Jeffery. It will always be around. And the resale (God forbid!) of a Dakota will be better than a wildcat.
The RUM brass is indeed much cheaper than Jeffery, so much so that you could lay in a lifetime supply for next to nothing, in case the RUM's ever go away.
Performance of the Dakota is very much on par with the Weatherby, BTW. So you can compare it to that, since the Dakota data is hard to come by.

This is a very sound idea, but I think you will find it easier to neck down and shorten the 338 RUM than the 300 RUM. The 338 starts out a 2.76 inches, so there is only 0.140 to go. If you compare the velocities of the 30-338 with the 300 Win Mag, you will at least get an idea of the FPS you gain in 30 cal pushing the case from 2.5 to 2.62 in a mag case.

Who cam make brass on special order ? Does anyone tried this before ? Any idea of associated costs ?

tks a lot!