Sorry to bring up an old thread but I have a model 29 that was my Grandfathers, my Mothers and now mine. I shot it some(close to forty years ago). The last time I shot it, when I took it apart to clean I noticed the firing pin was broken. So I put it away(for forty years).
This winter I took it to the local gunsmith to replace/make a new firing pin. He ordered the part. I took a couple of months for it to arrive...
When he replaced the firing pin, he ran into the same issue you described. All of the rounds spill into the action when pumping a new round. He found a very fuzzy diagram online that showed a ball bearing but doesn't say where it goes. I guess I didn't see it fall out when I took it apart many many years ago. The diagram shows multiple options for what I assume would be the Model 20, 20A & 29B so I wasn't so sure the ball was originally in my rifle(but now I know it is)
The gunsmith couldn't figure out where it is supposed to fit. He let me come into his shop and see if I could figure it out as it was frustrating him. In the three hours(over two different days) I spent playing around with it I couldn't figure it out either.

I have it back at my house now. My question is, can you(or any other member) explain or post a photo or two of exactly where that ball is supposed to fit? It would be greatly appreciated!

Thanks for any help and have a great day!
Bob