I have always wanted a COP. I own an old Berreta Minx in 22 short with the tip up barrel. I have also shot a Krag. It was smooth.Nothing too terribly exotic, yet.
I have shot a .357 COP. That was neat.
I'm quite sure I've also shot a Luger.
The little tip-up Beretta autos and their clones are "unique". I've shot the Beretta .22LR one of those (Tomcat?)
I've shot an old Krag in .30-40 and a reproduction Sharp in .45-70. The .45-70 is a cool cartridge, for sure.
From an operating standpoint, the H&K P7 has probably been the most unique.
-J-
1928 Colt Tommy Gun (with factory letter saying it was sent to a WV coal mine...)