I'm just curious as to what purpose the old tube magazine serves on these shotguns? Is it still useable? The 870 has the standard tube, and the 590 appears to have the extended mag tube.

I believe they still need the mag tube in place to stabilize the forearm as you cycle the action. It also needs the threaded tube cap to keep the barrel secured into the receiver.

I doubt that the tube is functional as the lifter required to raise the shells out of the tube and up to the chamber would be in the way of the magazine to feed the shells.

The extended mag tube on the 590 may be to help balance the gun since all the rounds are now sitting back and below the chamber area, but that is just a guess.

I can say for sure the 870DM tube is not usable for loading. But its still used for operation of the foreend. Not sure exactly how you might imagine one would use the gun if it were not there?

quote:

I believe they still need the mag tube in place to stabilize the forearm as you cycle the action. It also needs the threaded tube cap to keep the barrel secured into the receiver.
I doubt that the tube is functional as the lifter required to raise the shells out of the tube and up to the chamber would be in the way of the magazine to feed the shells.

Yes, those are the only two functions of the mag tube.
There is a pin through the receiver end of the mag tube on the 870, so you couldn't put a shell in it anyway.