From the adage of buy low and sell high, there is very little investment potential in MGs any more, those days are long gone. Outside of historically important or factory models, prices are too high to look at this as investment. If you didn’t get in during the mid-1990s “awakening”, you really don’t have much to work with.
Facing facts, there are very few people who can afford to pay $40,000 for an MP5 conversion today, what used to be $3000 in the early 1990s, and that should be included in any evaluation of investment potential. The true investment will be at the low end. MAC-10s, other formerly low-end, but factory, MGs, these are more affordable and hence will be more in demand to a much larger market.
Don’t buy M16s or HK conversions thinking you’re going to make money on resale, you really wont. The action will be at the low end.
In terms of stock market, the casual investor will not be making any money buying Apple or Google stock at $250+ per share unless they have the funds for big volume to capture profit on small price increases. Casual inventors will be looking for lower-end stocks with more growth potential. The MAC-10 approach.
ETA: Basically, if you were a Machine Gun News guy you did ok. But if all you know is Small Arms Review, then you were either too late to the game or you were blessed with lots of disposable income.