Most common insurance carriers will provide coverage for reasonably priced legally-owned firearms, scopes, cases and accessories through a Valuable Personal Property clause or separate policy. Coverage varies with the company and with the premiums you are willing to pay. When I say "reasonably priced", I'm not talking excedingly high dollar items, but it certainly can include custom rifles designed for hunting, for example, that are not unreasonably priced far from mainstream pricing for other similar items. Cameras, jewelry, computers, art work, and other similar items are in the normal type of items they will insure in this manner also.
If you are a collector or have unusually high value firearms well beyond the mainstream norm, say values with medium to high 5 or 6 digit values and beyond included per insured item, then you will need to find specialized insurance. Most insurance companies can provide you with references to do your own shopping. Believe it or not, people have been known to pay well into the high 6 digits+ for certain collectible firearms.
Just know that some of these specialized insurers provide coverage with "strings attached" that limit and define how and where, for example, you can store the items they insure. I find what they require is not far beyond what I would do anyway considering the value of certain items they are insuring. Other restrictions may also apply, but for the sake of some PM posters, I'll leave any further issues undefined and let you discover some of them yourself. These specialized insurers will insure your Ferrari collection also, where All State or Geico will probably pass due to item by item appraised prices.
HTH.......