Look down on the top of the receiver with the loading gate open.  The angle of attack for the cartridges is not ninety degrees from the axis of the bore's center line.  It is a bit more open let us say 100 degrees.  I do not know the correct angle.  I thought about this, too, in the past.  I took five cartridges and carefully dropped one at a time in the loading gate.  Each successive cartridge was always a rim thickness behind the previous one.

Even using the Parashooter/Steinmayer Deluxe Krag-Jorgensen loading charger I could not create rim lock.