20m really isn't enough to test hop up properly. A basic amount of hop will get you that far, and issues that aren't severe (like everything moving around) usually occur after 30m-40m. It should be getting to at least 70m stock. If you really want to waste a bit of money, and you don't have an easy range to measure out, a rangefinder can be had for about £100, so you can check real distances. though estimating on google maps on your local field is a cheaper but more difficult option.
I do second Rogerborg's opinion that the hop rubber got lubed, which would have some pretty bad effects on the hop up.
One thing, if you haven't tried, is to check air seal. Plug the EMPTY barrel with a finger (empty that feed tube, be 100% sure) and fire it. It should at least cushion the piston, and not get the dry fire slam. Or if it's very good, you should be able to hold a bit of air pressure and hear it release when you remove your finger. With the consistency it's probably not this. but it's worth ruling out.