Very few guns appreciate significantly. That’s been covered pretty well. Compare goods to similar goods over time and you figure out that its all relative.
That said, there are guns that do appreciate quickly, generally due to something less than fortunate. Right now TC stuff is hot because the brand has basically been put on ice with all tooling being used for other purposes and the brand name being up for sale. There are still TC fans out there driving the prices up for what’s available. The things that seem to be jumping in price are things that are short lived but very interesting. Hudson H9 as an example. Was HOT stuff the second it was announced. I don’t know what happened to the company but they don’t seem to be putting out the pistols much if at all, and prices have been crazy. That will die down though when they are either a has-been, a never-was, or the next big thing pops up for people to go wild over. Short term, you could possibly do well on oddball stuff like that, but long term your missing the peaks and settling for the averages, or even the value valleys.