Let me add, be sure to have the rim of the round you are inserting in the magazine in front of the rim of the round below it. It is pretty hard to have the rim catch and hang things up when it is already in front of it.
While this is helpful, it's actually not mandatory with a Mosin-Nagant. If the rifle is loaded correctly, the interruptor will allow it to feed properly regardless of the position of the cartridge rims.
This is important because these rifles were intended to be loaded more or less exclusively with stripper clips. It's important for stripper clips to be symmetrical so a soldier doesn't have to worry about inserting them upside-down in the heat of combat.
Also, loading a Mosin-Nagant with stripper clips forces
you to push down on the rear of the cartridges. Pushing down on the middle will make the rims jam sideways in the clip. Therefore, if you use the stripper clips properly, the interruptor-ejector will work properly.
I just wish Sergei Mosin had designed the clip to be self-ejecting like on a Lee-Enfield.