I'm into collecting in several areas, all early to middle 20th century Americana. In the supply and demand equation, you should weigh demand more heavily. The 1970s days of the buckskinner craze for young men is over. Back then, there was a back to the roots movement that helped keep demand high. The re-enacting Revolutionary and Civil War battles is also getting less popular. History is barely taught in schools, very few historic movies come out, seldom very popular with the young 20 year olds. All that hearkens to lower demand for original long rifles.
There will always be SOME that are into history, no matter how esoteric. The best items will stay somewhat valuable. Interestingly, I'm right now selling a rare, early brass lens, the type used by Watkins and Brady and other wetplaters. It's worth about $8-10 grand. The only high rollers contacting me about it are in China and Korea. I have 2 50 year old classic Honda motorcycles, barely worth what a set of wheels goes for on a modern Harley.
Colonial everything is down, primitive art, fine art, furniture, household items, pottery, coins, anything you can think of from our early history. If you bought high 15-20 years ago, you are going to be left holding the bag. Strangely, industrial things from the 20th century like giant oil and automotive signs are up. Muscle cars...down.
Like the guy above said, the lower prices allow me to get a few rifles, because I love history. I'm glad I'm not competing with those that were buying them for $50,000 a few years ago. MANY of the guns I love are now sleepers, going for very cheap prices. Those I collect, I've always bought what everyone else ignores, and ignore what everyone else buys. If you do that....you can acquire a nice collection. But everyone knows guns are/were valuable. You don't often walk into an antique shop or estate sale and "scoop" one up cheap, like you can other antiques.