Call Eben Bull or Fred Schertenleib at Estate Management Services. Honest and competent. They know where to find specialists for all the different kinds of collections people accumulate: china, furniture, pocketwatches, jewelry, model trains, guns.
I have used Bonhams for general antiques. Their rep lives in Carefree; very sharp woman.
Consider consigning all the firearms to an auction house, like Rock Island.
I have also used Legendary Guns (19th Ave.) to appraise guns. They charge a flat fee per gun. When acting as an appraiser, they will not buy anything. It's unethical for an appraiser to also be a buyer.
Sell only the whole collection; don't let somebody cherry-pick and leave you with the hard to sell dogs.
Whether selling at auction or to a dealer, the heirs should expect to net around 50-60% of fair market value. Both businesses have high overheads, and need to make a profit.