I vote with Will Beararms that the "fore-end tube nut" has come loose. Be careful tightening it with needle nose pliars. You can get a wrench on Ebay or a better one from MidWay.
I replaced my forend using the 870 "slide assembly" for a wrench.
If the tube nut is tight, is it possible that the action bars of your tube assembly are worn out. If so, the striations on your magazine will kind of curve as opposed to being staight.
I don't really know what I am talking about, but read this somewhere.
Last edited by TheKlawMan; April 6, 2011 at 03:23 PM.