I dont know much, nor did I sleep in a Holiday Inn Express last night... so take this with a grain of salt.
Investments should be diversified.
A weapon, is an investment in your future, in your security. It may not appreciate in value much, if at all, but it might save your life. You should have what you feel you need.
As comparisons to monetary investment vehicles available today.... no way.
I think even pre-86 machineguns are a bad investment. Sure, you can look at the guy who bough at the perfect time in '85, and look at it today.... but you could do the same in stocks and have a MUCH higher return. We have to live in reality.
Machine guns as investments could be gon, as stated, with the stroke of a pen. And it very much could happen.
What you need to study, is the long term viability of weapons, vs something else. Using the rule of 72, your money doubles ever 12 years earning 6%, or 6 years earning 12%. What is typical is somewhere in the middle.
So - will the AK/AR double in value every 6-12 years? Not likely. Will a machinegun? Doubtful. Sure - they will continue to rise, as demand increases and supply is zero.... but very risky for little reward.
If you want guns, buy guns. If you want to invest, start with some decent mutual funds.