broken magazine tube

[gaalfred]

I have a 20 gauge 870 that has started double-feeding shells into the receiver which requires dis-assemble every time to remove the shells. I replace the left side shell latch (number 55 on the parts diagram) thinking that was the problem. It still does it. Many forums suggest that I adjust the shell latch so it catches the shell, yet releases it when the action comes back. I noticed that the shell can move side-to-side in the magazine tube far more than the width of the catch that should be holding it. In comparing it to my brother's 870, I see that the shell follower on my gun comes all the way out until it engages the first latch on the right side of the gun. I also notice that the metal on the upper part of the magazine tube looks like broken cast iron, not like machined steel. I believe the rear end of my magazine tube is broken off and gone. I have searched for several hours online and cannot find one reference to this problem. It appears that the magazine tube is silver soldered to the receiver and not repairable by a hack such as myself. Anyone every hear of this happening before? Any idea what the repair would cost? This gun is probably 40 years old, but has not had a lot of use during its lifetime and should not be broken as it has never been abused. Thank you for any input you might have.

[Synchronizor]

Pictures would help.

Is your 870 one of the old-style 20 gauges built on the same large-frame receivers as the 12 & 16 gauge guns?