My grandparents set up something that I think is brilliant. When they are both dead, an appraiser will come in and assign a value to everything. Once that is done, the value will be divided equally and each child will be given "money". Then each item will be auctioned to the kids. If someone wants something bad enough, then they will bid until they get it. Once all the "money" has been spent, leftover items will be auctioned in an estate sale. The kids will have the opportunity to buy items at the real auction, but they will have to spend their own money. After the auction, any money made will be divided equally. Leftover items will be donated to missions.
While I commend your planning, unfortunately I've seen this play out several times and someone always feels "slighted" and it creates negative undercurrents within the family.
All my firearms will be left specifically to my son and wife, everything else will be left to my wife to pass on to our children as she sees fit.