How does gravity take a slide upwards ?
Turn the projector up side down...
Jams usually occur when the slide is going in, with the powered arm of the horizontal fed projectors forcing the slide in. With the gravity fed slide, it just stops when it doesn't want to go in. When you turn it up side down, it just falls out! - OK, but the arm that pushed the slide out usually works - there are times when the slide is fully in too long, has expanded or buckled/melted, and gets jammed... But in those cases, then manual slide projector won't help.
I like manual slide projectors for meduim format, mostly because I don't have as many as 35mm. I know people that would have no problems jamming a manual slide projector, they have a tendency to force anything that doesn't want to move...
I have a P11, the rubber roller is always the main problem (it dries out and gets hard), once that is refurbished, it works well. I've been planning to convert it to an LED light source. But I'd like to get a 6x9 projector... (I have 6x7, 6x8 and 6x9 transparencies)