In the yast handful of years, machine guns did basically the same as the housing market. Prices climbed a lot in the early 2000s, and they dropped when the economy tanked. People just didn't have as much disposable income, and machine guns couldn't bring the same prices. Also, a lot more owners put guns on the market to pay bills. I bought my Vickers last year for 60% of what the previous owner paid in 2001. OTOH, the prices from 1985 to 2005 increased by 2000% or more.
So, I think this would be a pretty time to buy, but as with anything you need to take the time to find the right purchase if you are buying an investment. Something that's undervalued or underpriced. I got the Vickers because it is multi-caliber capable, it was a C&R gun, iconic, and cheap. Something like a Thompson is probably still too highly valued now to be a good investment.
Some of the better SMGs like Sterlings might be good buys, or odd variants of things like Madsen LMGs. All a matter of what you can find at what price. And of course, if the economy re-crashes again in a year or three you'll wish you had waited until then.