In 1963 I took my 50 cents/week allowance, saved up, and bought a $2 Shooter's bible. It shows the retail price for most guns that year.
If you go to the gun shows and pawn shops, you probably have a feel for what guns are worth today.
I have derived a simple formula for compounded interest rate for a scientific calculator,
You may have to do 2nd y to the x to get a root.
% = 1- [root number of years[$now/$then]]x100
If I do this, almost all gun are 3% compounded, over 10 years, 50 years, or 100 years.
That is the same 3% rate for name brand guitars and ounces of Gold.
It is background inflation.
There are two exceptions I see in the 500 page book. Colt revolvers are more like 6%. Mossberg 16 gauge bolt action shotguns are more like 1.5%
There is an illusion about the appreciation of bolt action surplus rifles. When Century or whoever are importing them, that year they may want to sell 10,000 of them.
If we look at a supply / demand curve we can see that the demand price is high for low volumes and low for high volumes. After they stop importing a surplus bolt action rifle, it is only available on the secondary market. There will be a dozen at the gun show and a dozen on Gunbroker. It seems to double in price overnight. The value of that rifle is going to change over time, and will find it's way to the 3% compounded exponential slope. That curve may not point back to exactly the import year price, but the looking forward slope will be right on 3%.
I don't consider 3% growth on guns to be an investment. That is so low, it is more like paying rent for me.