I have State Farm insurance. I brought it with me when I moved here. They're used to handling firearms, but they wanted a lot of documentation when I tried to add my collection. Most concerning, they didn't like the fact that I used part of my collection to teach firearms safetey. They said they couldn't insure those guns because they were part of a business.
The standard insurance companies are too expensive for a rider in comparison to others that specialize in such insurance.
Check out any of the collectable insurance companies. Many only want the dollar value of your guns and they will cover them. Last time I plugged in some numbers, it was around 150 a year for 10K of insurance.
google firearm insurance, gun insurance and collectable insurance.
A search here will give the OP a few other options, that will save MUCH money vs a rider on your homeowners. Also in the Northeast, many homeowner insurance companies are antsy about guns (or certain types of popular guns) demanding lots of info on exactly what you own. Queries to some companies may lead to cancellation of your homeowners. Personal opinion is better to have a special policy with another company for guns here. [I might think differently if I lived in TX, but MA is NOT TX that's for sure.]
My wife is my agent (home, auto, etc...). She said lots of people have guns, and just "bringing it up" that you have guns won't raise your premiums, etc.
Suggest you call your agent and ask them about adding a rider (i.e. for theft, fire, etc.). You may need to show receipts or appraisals so they'll know what they're worth.