Thanks Pettifogger,
however the rifle is an original Colt Lightning Baby Carbine in .44-40 (barrel length: 20 inch - serial number: 84160 - it has no safety lock: I mean that it can be pumped back and forth irrespective of the hammer position). The problem: when I pull the pump backward and a new round comes out of the magazine, it goes onto the cartridge carrier in a wrong way, i.e. not horizontal but oblique, with its rim up, which makes the rifle jam. Sometimes (rarely) the problem does not occur and the rounds come out of the mag in the proper way (perhaps when I shut he pump forward violently?): in this case I can discharge the whole magazine by pumping back and forth without never pulling the trigger. The reason why I believe that my problem is similar to Ronny's (either the ramp or the pump slide, or both, must be worn out) is because all the ejected rounds are deeply engraved by the cartridge stop both on their rims and along the cases: I believe this means (as you said in the post) that the top of the stop finger is dragging onto the cartridge rim (and the rest) as it is trying to follow the bolt in its run backward. Since it is not possible to intervene on a worn slide, there might be a hope in hammering the ramp behind the finger, to make it more protruding, so that the slide can push down the ramp sufficiently in order for the finger not to touch the cartridge rim when it comes out of the mag (and not to deviate it).
If this is the problem, I should know how and how much to hammer the ramp (as Ronny did).
I hope that, in spite of my poor english, I was able to describe the problem to you, whom I consider the Lightning Wizard!
Thanks