The question remains: What do intend to use the rifle for ?

Serious stalking could be hunting Bear in thick cover OR trying to sneak within 300 yards of a Mountain Sheep.

You are correct about a 28" barrel on a factory rifle.

A 300 RUM kicks a lot. When they first came out, I bought a 700 Remington BDL for that cartridge. I fired 7 rounds and took it back to the gun shop. It is much worse than a 375 H&H or a 300 Winchester Magnum.

I can't think of any game in Spain that would require a 300 RUM.

I have a 338 RUM for long distance target shooting but it has a 36" barrel and weighs 20 pounds. Even it has a sharp recoil !