Like is said above, most homeowner's/renter's insurance will cover firearms up to a set amount. Mine covers up to $1,500. It will only cover theft and total destruction (like from a fire).

If you need additional coverage, you have several options:

1) Some insurance companies will allow you to purchase a "rider" for additional coverage.

2) You can purchase additional coverage from the NRA.

3) You can purchase a "personal articles" policy from your home insurer (if they offer it). This usually involves an appraisal. It is also much more comprehensive than the standard built-in coverage. It will usually cover damage. I would reserve this kind of coverage for expensive or collector grade firearms. A good rule of thumb is that this kind of coverage will cost $1 a month for every $1,000 it covers.