The Express will take just about any 870 barrel.
The Express will accept most any accessory, including different stocks and magazine extensions, BUT.........
There are two lugs, (commonly called "dimples") in the end of the magazine tube.
These lugs are to lock the new style magazine spring retention system into the tube.
These lugs must be removed before an extension can be used.
With the lugs out, you can install an extension.....BUT this makes it difficult to return the gun to the standard magazine length, since the lugs are needed for the new style spring retention assembly.
It's possible to return the gun to the standard magazine length, but this requires some "work arounds".
The lugs can be drilled out with a 1/4" drill, ground out with a Dremel tool, or pressed out by using a mandrel of some sort.
