While sizing up the 45/70 cases for consumpter I tried on purpose to get a rim lock with my 35/40 cartridges. It is quite hard to do, aiming the barrel straight up and closing the magazine door slowly I could get it to lock up. When this does happen the front of the cartridge goes down instead of up into the chamber. I did not try really hammering the bolt handle to see what would happen.
Laying a couple of cartridges in the magazine with the gun level and snapping the door closed, the cartridges did right them selves and no rim lock occurred. The process is unseen and I can not explain why it happens.  With windows into the magazine and high speed cameras maybe you could understand what was going on in that dark place. Without the technology to understand what is happening, it is sort of like magic.

Parashooter's  drawing pretty much explains it with original length cartridges. With shorter ones who knows.