I'm not an expert by any means. I can tell you though that I had a Ruger 300 win mag that had a factory 24 inch barrel. I think all Rugers 300 win mags are that way. Even though Rugers aren't my favorite rifle, I can't see them making all of their 300 win mags wrong.

I also had a Winchester 300 win mag that had a 26 inch barrel.