This subject has been discussed not too long ago but it's 01:48 and nothing I care to watch on TV.
So, my solution to this "problem" is selling everything myself, I know what it's worth, what I paid for it and what I should be able to get. The hard part, very hard, is that everytime I sell off a gun, I sell a part of myself, I try to find them good homes because I really want the guy that buys them to enjoy them as I did and I always take a loss but at least I know they are in good hands.
My last sale paid for my 2019 property tax so in essence, I'm keeping a different inheritance safe for whomever.
I'm keeping practical guns now, their values are quite easy to research and I will have them listed on my Will, itemized as to how research their value once I'm gone. I doubt I have more than $5k in my guns anymore and if "they get" $2500 bully for them.
So I say, unless you have a massive collection and specialty guns, sell them yourself and reinvest in real estate.
Otherwise list everything in your Will, what you paid for it, how to research current prices and let your inheritors worry.