If I understand you correctly about the flare at the end of the tube, then it probably is open just enough to keep the mag from going down all the way, and might be allowing the follower to come out to far. The flare (if you mean a swell in the tube's dia.) would cause problems with the threads fitting back in the receiver, and someone may have tried to file or grind down the threads height to make up for for it, or tried to hammer the swell down. It sounds like you need a new magazine tube. The mag tube might have been taken apart incorrectly and the follower was knocked out from the bottom instead of letting it drop out from the top.
Also, take the magazine tube apart and clean the inside. The spring could be gummed up with old oil and rust. Plus there might be a home made plug inside the tube that is not allowing the spring to expand properly. The follower could have rust around it , too. It's not hard to take apart, but it is tricky enough to make the steps hard to describe. Check the net for any videos on the matter, or bring it to a gunsmith that was working on shotguns during the 1970's and 80's as he will know the model 12.