Meta sorts accessories by what they do to the gun
Meta's weapons page gives gun accessory sellers a fairly specific list. Holsters, pouches and magazine holders can be promoted when targeted to people 18 or older. So can gun cases, safes, locking devices, paint coatings, gun slings and tripod mounts. On the other side, firearm parts, silencers and suppressors, optics such as telescopic and iron sights, and products that enhance or customize a weapon's form or function are prohibited. Safety courses for firearm training or licenses are allowed.
The pattern in that list helps when you meet a product it does not name. Things that carry, store, lock or protect a firearm sit on the promotable side with an 18+ audience. Things that change how it aims, fires or performs sit on the prohibited side. Sort your catalog into those two groups before you brief any creative, and treat anything that falls between them as prohibited until you have read the policy page yourself.