How do you take the barrel off a Sears Roebuck model 300?

First, unscrew the magazine tube cap, located at the end of the forearm. Then pull the action back to about half-way, then you should be able to pull and jiggle the barrel and forearm free.

To reassemble, slide the barrel and forearm back on, and pull the action back until the barrel slides into place, then rescrew the magazine tube cap.