Welcome Siggy.
Two distinct criteria to your question.
1) Will this "x" firearm be worth more or less in the future.
2) Will this "X" firearm be priceless to my great grandchildren regardless of brand. (heirloom).
For example, my Grandfather's Mauser isn't worth a lot of money, but is priceless as a family heirloom.
However, my 1978 Ruger Blackhawk in 98% condition, will probably be worth more money in the future.
Off brands, all depend on uniqueness and demand in the future. Name brands have more produced, but are collected at a premium. Example. Pre-1964 Winchester Rifles, S&W Revolvers like the Model 19, Schofields. etc,.. Yet 30-40 Krag rifles are good, but have not increased in value quite as much, except to the family members that cherish them.
Check the price of a Colt Python lately? Ka-ching! M1 Garands are jumping up. M1A's etc,....all will be both heirlooms and increase in value over the long term, like many others.
As a collector of antiques,.. pottery, furniture, guns, etc,... There are variables. I don't know how much a Glock21, like mine will be worth in $$$ in the future. BUT I do know it WILL still work...

Just like my Grandfather's Mauser.

Interesting question.
I agree with Tom, but my son can figure it out after I'm gone....