I've been a 7 Dakota
user for about 3 years or so. I would consider it to be a ballistic twin of the 7 STW since the case capacity is within 1 grain. I got started on it when I bought an in-stock custom rifle
. I was asking around for an available rifle in 7 STW and got steared toward a great rifle in 7 Dakota.
I buy the Dakota brass through Midway...around $26 for 20 pieces if I remember. I don't shoot brass all that long but I think you could get around 20 firings if you weren't pushing pressures too high. Initial case neck runout is around 6 or 7. I once challenged Don Allen about this and he took some of my high-runout returned brass and fired a .3 something group with it.
I bought 200 pieces from someone on LRH.com a couple months back and the runout was around 2. So maybe I just got 6 or 7 unlucky lots over the 3 years?? Anyway, the initial runout doesn't matter after the first firing. The cases also seem to be too short for saami specs. They always stretch in base to shoulder length about 15 or 20 thousandths at first...and this is with rifles chambered with reamers from 3 different sources.
With a 176 grain Cauterucio (.750 bc) and 76 grains of H1000 I get between 3100 and 3150 with a 26 inch barrel. It varies a little depending on the barrel. These are fairly hot loads. Minor casehead marks and an occasional sticky extraction.
I never considered the .308 version because I have always been a proponent of using the best bullet BC and the .284 is quite superior to the .308 given the same bullet weight. And of course, you can drive a given weight .284 bullet faster than with a .308 bullet.
Don't know about the case forming question.