They don't ask for serial #'s but individual guns worth more than $2500, you have to itemize those and in some cases provide documentation for. If you are declaring a gun with a scope (e.g. a $1000 long gun with a $2000 scope, like a nightforce/etc) they will want more info for sure, as the declared value way exceeds the value of the gun when they check it against the book.
The good news is it covers accessories. For a belt fed you can tack on the tripod, cost of belts, spare barrels, etc. For an AR you can add on the cost of the optics, or whatever.
Which makes it pretty good; my old homeowners policy wouldn't cover accessories unless they were attached to the gun, and everything else got lumped together in to a "sporting goods" rider which meant most of the stuff wasn't covered at all. (Magazines, whatever).
The only thing they will NOT insure is ammo.