300WM ammo is available everywhere I have ever hunted, so too is 7RM, which I have not ever used, but my buddy and I were in a situation many years ago that all our gear was lost when our boat capsized, the rifles weren't lost as they were on the first trip.
We were both able to buy ammo in a tiny town in the middle of nowhere.
The same is true today, we could buy ammo again for those 2 rounds if it had happened today and not 25 years ago.
We were lucky that 2 days later most of our gear was recovered, only a camera, spotting scope and tri-pod were lost, even our ammo was recovered.
If building a custom, I would recommend a barrel length of 26" minimum and optimum length at 28" if you are shooting to LR distances.
I have a 300RUM as well, but factory ammo for it at this time is not only super expensive, but also down to ONE brand here, that being Barnes TTSX @ $130-$170 a box. I know I won't find that ammo at the same store where I had to buy ammo all those years ago.
It all boils down to being pressed to shoot what's available.
Many cartridges fit the bill, 30-06, 300WM, 7RM, 338WM, 7-08 and 280AI.
Supply can be spotty for all mentioned.
Cheers.