For the purposes of the media, any time a plural noun can be used to describe someone's firearm ownership, it's an arsenal. While this is technically correct, so too would the use of the word collection as Brian Pfleuger pointed out, but the word arsenal just sounds scarier and makes for a more attention grabbing headline. Kind of like when someone gets arrested with a joint, $100 bill, and a .22 Rifle they always run a headline something like "Drugs, Guns, and Cash seized in Police Sting!!!" Yeah, it's technically correct, but it gives a very different impression than what actually happened.