I have a 187A that belonged to my grandmother. My recommendation is that, if it is operating just fine to leave the mag tube alone. The feeding on these things is pretty intricate and a bit finicky to get set just right. Once you take the barreled action and trigger assembly out of the stock, you will see that the tube is kind of fixed and worked nito the whole feeding and fire control parts under the receiver by a retaining screw in a dvetailed block. Other than that the tube is held under the barrel by another dovetailed fitting. After disconnecting from the works in the fire control area, you would just drive the dovetail out with a drift and hammer.
I still haven't been able to get my grandmother's springfield to feed other than manually. The number of parts, in the feed and firing system is pretty rediculous once you tear down a Ruger 10/22. timing and spring tensions have to be just right.