I think that basically is what the 7mm Dakota is. The only difference in case would be that the base of the 300 RUM is .550 compared to the Dakota's .544 and the rim diameter(RUM) is .532 compared to .544. Other than that the two would be so close, I would just save all the trouble/money and buy the 7mm Dakota. If that's the performance level that your looking for.
The reason I posted this question is because I've heard bad things about Dakota brass. Like the brass being undersized causing shooters to get excessive amounts of expansion. What I want is something that will run with the 7mm STW but without the belt and long case. Currently have a winchester laredo in 7mm stw that I'm rebarreling because 162 gr a-max will not fit in the mag when seated to touch the rifling. C.O.L. over 3.75"
It's true in my experience that the 7mm Dakota brass is small. I can't remember right now but I think the brass expands about .015 in headspace on its first firing. However, I get quite a few firings from it...maybe 10.
Also, in various rifles chambered in the 7 Dakota, with the 176 grain Cauterucio, I have gotten between 3130 and 3250 fps with a 32 inch barrel. Your mileage may vary.
I guess I will go with 7mm rem mag. But man I sure would like to get rid of that belt and get that extra 200 fps. Just curious, what kind of groups were you getting with the 7mm Dakota. The barrel I'm gettting is a 28" Krieger with 9" twist.