Got me a Savage/Stevens model 620 12 gauge riot gun.It's really, really old. Can't find a serial number anywhere.

I'm having trouble loading the first round into the chamber. I assumed you put it into the magazine and rack it into the chamber using the slide. But sometimes when I do that the shell gets jammed up in the receiver. After I fire the first round it works perfectly.
Shell jammed in receiver
Soooo....... Am I supposed to just put the first round into the
chamber and then put the rest into the magazine?
......I am so confused.......