At one time it was questionable. But in recent years I've not seen any store that carried 300 WM that didn't carry 300WSM also. It may be a regional thing. But 308, and especially 30-06 are going to always be easier to find, cheaper and with more options. I don't think there is any danger of the round becoming obsolete. It is too popular for that. But since I hand load it is all pretty much even for me.
I have a 300 WSM. I really don't need it, I find my 308 does at least 95% of what I need a rifle to do. But I just like that rifle and have come to respect the round.
The problem is that very few rifles are being made to best utilize the rounds advantages. Depending on bullet weight you can get 200-400 fps more speed than 30-06 and come within 50-100 fps of 300WM. But with recoil that almost exactly splits the difference. You can put together a 7-7.5 lb 300 WSM that won't beat you up too bad. A 300 WM much under 8.5 lbs is starting to get my attention when I pull the trigger.
But no one other than Kimber is building 300WSM's any lighter than a typical 300WM. And you have to ask,do I really need the extra 200-400 fps?
