A person who collects firearms and - often militaria as well - relating to certain historical periods. This person may also be into shooting said firearms but this is often secondary to appreciating them and finding out arcane bits of historical trivia about them. This main pursuit is usually done by buying large expensive reference books about the guns in question and swapping info and expertise with other gun dorks at gun shows, meets, and on-line forums.
Gun dorks dislike being called "Gun Nuts" as they beleive this incorrectly lumps them in with those of the Glock-AR15-AK-shootupthecountryside crowd.
"That guy has every book to go with his WW1 rifle collection - he's a real gun dork!"
"Yeah, he's into guns but not just any guns, specifically Austrio-Hungarian Jager rifles - he's a Gun Dork not a Gun Nut."