I'm posting about my Remington 870 Police Magnum. This gun is new to me and I treated as a project gun. I received it as a police surplus gun in perfectly serviceable but worn condition. It was given to me by a friend of mine. He also gave me a Choate Tool 2-shot mag tube extension.
First, a look at the finished gun - I duracoated it in Magpul FDE, dressed it with Magpul furniture and a Mesa Tactical Picatinny rail, and some other stuff.

Turned out really nice. Now, the issue I have is that I can't feed but 5 total shells into the mag tube. Before we go in the wrong direction with this, an important note:
The mag tube is not dimpled. So, no need to drill them out or flatten them.
Also, I read this advice: http://www.aiptactical.com/Page_2.html at AIP Tactical about popping the extension with the heel of your hand to make sure that the spring doesn't bind up in the tube. So I've done that too.
Here's the Choate extension screwed onto the mag tube.

Here's the extension removed from the mag tube along with the spring.

Here's the GG&G enhanced follower.

Finally, here's the spring laid out next to the barrel so you can see how long it is.

First thing I did to troubleshoot was to remove the spring completely and make sure that the follower and the shells would slide smoothly into the extension, that they weren't binding at the seam between the extension and the mag tube. And they do slide smoothly through. Nothing gets hung up. That seems to leave only the spring as a possible culprit, does it not?
I noticed that Choate Tool sells the mag extension kit with an orange guide rod of some kind. It's not a "plug" to reduce capacity for hunting (I don't think so, anyway). But I don't hav this guide rod. All I have is the spring and the mag tube extension itself.
Does the spring look too long? Maybe cut it down?
Maybe I need a new spring? (What could be wrong with a spring? It isn't perfectly straight, I guess.)
Ideas/suggestions greatly appreciated.