Also with a real shotgun, once the first round is chambered an additional round can be added to the tube, meaning that in total the shotgun in-game should be able to hold 6 shells in the tube and one in the the chamber, for total of 7.
I have not tested whether this is the same case for the Repeater, but i believe this all applies to the Repeater as well(correct me if I'm wrong gun people, I've never owned a lever action personally).