It's the only one of the WSMs that gained any appreciable velocity over the basic standard length cartridge of the same caliber.
There is an ongoing debate over why that is....
Pretty sure the 300 WSM makes appreciable gains over a 30-06, like 200 to 300 fps in just about every weight and results in 500 to 600 more lb-ft in every weight. I could be wrong but I'd call that appreciable.
The proper comparison is to the .300 Win Mag - the cartridge the .300 WSM sought to duplicate in a short action. Which is why it's called a .300 WSM (not a .30-06 WSM, which wouldn't make sense anyway since the .30-06 is not a Winchester-developed nor a magnum cartridge).
The 270 WSM is an awesome round and one I would like to have...
The 300WSM is an 300H&H IMO so... why?
Why not? Instead of needing an extra long action, it works in a short action. Being more efficient, it will do the same work with less powder, which means less expense and less recoil.
It's like getting something for nothing.
All of the short magnums are there for a good reason, they all do something that the old style magnums cannot do, or cannot do as well. Usually this is a result of the modern case design being notably more efficient.
The 325 WSM for example, is so efficient that it provides much the same performance as the enormous 8mm Remington magnum - and with a shorter barrel. - Nothing but a short magnum will give you that kind of performance in such a small package.
I like the older style magnums too, they are not going anywhere - but a lot of shooters are going for the short magnums these days, and with very good reason.
* Uses less powder to do the same job.
* Fits in a short action.
* Does not require as long a barrel.
* Kicks less while doing the same job.
The 270WSM that we are discussing is going to be very hard to beat for a mountain rifle for example, whicht must be light and handy, while also being powerful and accurate.
My best five-shot, 100 yard group so far with a 270WSM is .317", center to center. With careful loading and the right gun, these cartridges are outstandingly accurate.