There is no "H&K SOCOM". That gun is actually called the Mk.23.
Yeah, but it's much easier for people to recognize the name "SOCOM" than "Mk. 23". I'm not a gun nut, and it's a much more familiar name for people to look at and go "Oh, that gun" as opposed to "What's that?"
Besides, who said it'd be out of character for Dom to have a peculiar affection for a weapon that's been replaced by smaller, better things? I mean... he IS going after Erika.