Re: 300 Dakota- vs. 300 win mag vs 300 Weatherby con\'t...
You have created some excellent responses that we all learn from. Check into the fact that there are two .300 Win. Mag specs, the factory load and a longer mil-spec load that was developed for the U.S. Navy. The longer over-all length load is the way to go, George Gardner tells me that accuracy is significantly better if the chamber is correct and you headspace off the shoulder instead of the belt. I believe that Black Hills Ammo is loading the mil-spec load but doubt that we can buy it, but it would be easy to duplicate.
To further complicate things, in my rifles the .300 WSM matches or slightly exceeds .300 Win. velocities with bullet weights up to and including 180 grains. Accuracy is probably similar, but I would put my money on the WSM being inherently more accurate. I believe that the factory gets the WSM performance by loading at pressure levels that are right up there, have seen primers blown and markings on case heads on hot days with factory ammo.
Since you are interested in hunting to 6-700 yards either .300 Magnum will do a super job, you are well within their capabilities for accuracy and energy dump.
The .300 Win. mag has lots of data, is easy to load for and get accuracy, and you can buy factory ammo anywhere. The WSM is does not have a belt which is a definite plus as far as accuracy and reloading goes, it is a very capable performer and it will enable the use of short actions (which I do not like, short Rems require a single stack mag box which limits how many rounds and also does not feed as well as Winchesters M-70 WSM action). I would get a custom rifle in .300 built on a Winchester WSM action simple as that.
Just to complicate your life, you have not mentioned the new .325 WSM, for the animals you mentioned this is a candidate - 200 gr. Accubonds at 2925 and superb accuracy. My GA Precision built .325 WSM drives tacks, kills bears out to 300 yards with one shot and does not kick as much as some light .300 magnums that I own.