The anti-gun crowd has made me think along alternate lines of safely leaving firearms to family members. It would not be a bad idea to put a stainless auto pistol, a few high capacity mags and several boxes of ammo in one of those burial tubes or make one yourself out of PVC pipe. A handful of those moisture absorbing packets might be a good idea too. Bury this in the woods on the back 40 where it will not be disturbed and give the location instructions to a trusted friend or two at most. The "Red Flag" laws make it necessary to take precautions.
Also, I can not say this loud enough--AVOID PROBATE!!! Probate will take as much from your heirs as a fancy funeral, avoid that if at all possible.