I am a claims adjuster (20 years) for a major insurer. I'll let you know my perspective.
Read the policy you have. Find out what if covers. Some policies have a cap for firearms regardless of peril. Others have a cap for firearms for some perils, but not for others. A peril is a means of loss, such as fire, theft, etc. For instance, my own homeowner's policy has a $5,000 cap on my firearms for theft, but no cap (except my overall contents cap) for fire. In other words, if all my guns were stolen, I'd get $5,000 regardless of their actual value. If they burn up in a fire, I'd get full value up to my contents limit - which is substantial and well in excess of my collection value.
If your current policy provides inadequate coverage, ask your insurer if they offer a rider. Some have firearms riders. Others, such as my insurer, have sporting-equipment riders that covers not only my guns, but all my fishing gear as well. Get a rider from the company you are with before you buy specific coverage from another provider. Otherwise, if you have a claim, you may get involved in battle between insurers over who is primary.
Also, check to see if your policy providers replacement-cost coverage or coverage for actual-cash-value (ACV). Replacement cost is what is sounds like: you are paid what it costs to replace your gun. ACV: you get paid the value of your gun. Understand that even if your policy has replacement cost coverage, it will likely pay you ACV until you actually replace your gun, then it will pay you the difference between ACV and replacement cost. This is called "hold back" and most insurers do it. This helps prevent people from loosing, say, guns, and replacing them with a new plasma TV.
So, check the policy you have. Read it carefully. Read the insuring agreement, the contents parts, the exclusions, and the exceptions to exclusions. Only then will you truly know if you need additional coverage.