I think that airgun collecting as a hobby, and the number of participants within it, will shrink over time, due simply to the demographics involved. In the 1950's/60's every kid had an airgun, and shot it anywhere he fancied. He started with a cheap Diana and sometimes eventually managed to get a BSA or Webley.

When he grew up, he aspired to own the air guns he either couldn't afford or wasn't allowed when he was younger. and suddenly he found himself with a few old airguns and became a collector! And his collection grew, and he shared his passion with other blokes of his age, he joined clubs, and more fellas bought old airguns.

None of that happens any more. Far fewer youngsters buy airguns, a) because they have so many other interests, mainly computer based, b) shooting them is more restricted, and c) society disapproves more than ever. However some of these lads (or lasses) may well start collecting, but it will be those airguns they remember from their youth.

Ergo, will a 25 year old today be interested in a 'long-tom' BSA's, when he has been brought up with electronic Daystates with on board chronographs? A few might of course, but not in the numbers that hitherto have collected these things.
As such the hobby may well decline as we all get older, sell our collections, and die! A bit brutal but it's gonna happen!

Values will decline, or at best stabilise, for all but the very finest and rarest of airguns, for which there will always be a healthy demand, as there is for the best of anything. We can see evidence of this already - the very worth Webley Mk.3, can be bought in excellent condition for £150 to £200 - much the same sort of money as dealers were asking in the 1980's!

And to cap it all, there is impending legislation, which will inevitably curtail interest from the aforementioned young people.

I do hope I am wrong, but these scenarios applies to other collecting hobbies as well - eg model cars and trains from pre and post war. Values/interest is down due to fewer people being interested in acquiring for example old Hornby locomotives?