I'm not so sure that the 425 Express is under patent as the NZ hunter bought his own chamber reamer and had the conversion done here in New Zealand (as I read it from his hunting stories). Maybe in Spain you have more restrictive laws than we do. Here we can make any cartridge we like and chamber a legal firearm for it. Even 50cal Browning.

Here is a link for some information on the 425 Express.

http://www.458express.com/build.htm