Here's a schematic for your shotgun.
http://www.brownells.com/schematics/...ge-sid161.aspx
In addition to the mag tube, there are other essential parts for loading and keeping the rounds in the mag.
Check out parts 11, 12, and 18.
They are the elevator for guiding and lifting rounds.
The cartridge intercepter and cartridge stop for controlling rounds coming out of the mag.
The shapes are critical, so be careful with messing with them.
Or you could wind up with rounds flying all over the place and jamming the works.