You do not need the european gun card to buy these in Poland. You can have shipped (like from https://saguaro-arms.com/) or just from a private transaction. You do not need the gun card to buy the powder in a private (or "gifted") transaction either.
>And this kind of guns requires skills to use
A percussion black powder revolver takes a little more skill than a cartridge one, but not much. It's also quite deadly.
>I do not believe that mass shooting is even possible with it
Yes if you ignore the entire 18th and most of the 19th century, during which masses of people were killed with these weapons.
>As I said the main barrier is that there is no possibility to impulse buy.
Private sale is unregulated [0]. Conceal carry appears to be legal without permit [0]. Just as here in US I can get a pistol legally in 5 minutes with no checks or registration or permit, same can be done for a black powder revolver in Poland.
Watch Paul Harrell explain these firearms, they're no joke and I recommend every Polish resident consider one for self defense.
https://www.youtube.com/watch?v=TCYaiRmcYVI
[0] https://en.wikipedia.org/wiki/Gun_laws_in_Poland#:~:text=Unl....