I have returned to the old and somewhat obsolescent break-action "springers" that require one open-and-shut action to place a spring & piston under compression; when the trigger is pulled, it compresses a small amount of air in a relatively short chamber. They are relatively low in power compared to most PCPs, and as the springs get heavier (for more power), they then become harder to cock, "bounce" more when the spring is released, and have a sort of "reverse recoil" as part of the operating cycle. They are much more sensitive to variation and ask a good deal of the shooter.
There are some that are pumped up by one or more strokes of a built-in air pump, and ones that take CO2 cartridges, etc. Chooing one requires a person to really sit down and decide what power and accuracy level is needed for their intended use(s), how much they can afford, and - not least importantly - how much care and dedication. they are willing to put into them.
A half century or so , ago, I worked on a high pressure air conversion for an M-1 Carbine. I had 3,000 psi air to work with and the whole thing was quite effective, especially on the noise side.
A bit of research turned up that there was sort of a gentleman’s agreement with the Fedz, in the years the 1968 GCA was being formulated, That they would not include air guns in the rules as long as they stayed relatively low powered and/or specialized. Divers might remember that US Divers had a super neat HP Air powered spear gun?
I decided I was a bit ahead of my time, shelved the project, parts of which are still laying around here, and let someone else pick up the slack. I would caution the current HP Air officianados to avoid getting too far out over their skis.