sounds like rust on the inside of the twisted mag tube. a cleaning and then run a ball of fine steel wool thru...does it hang up? that is usually rust. there is alos a mag follower, sort of a cylander thing with a hole in it. the spring rides in the back of the cup and the cartridges push on the front of the cup, if it is rusty or cruddy then it sticks.
Sounds like the first thing is to take the tube off, then get the spring out and the follower; and clean well, then light oil, I use CLP.