308 is over pressure. They are not the same cartridge. NATO is designed with the Spanish Mauser's in mind. 308 has no such concerns and seeks maximum performance. You can fire NATO in 308 rifles and can probably get away with firing 308 in most NATO rifles but not recommended for the Spanish Mausers.

Mitchell's Mausers are in the $500 range. Real nice if you can get one for $300 or less like I did.