I know I've seen an in depth guide with pics somewhere, but I can't find it at the moment.
To get you started:
-Remove the rear grip.
-Remove the two screws from the fore grip.
-Use a fine slot driver to split the back end of the fore grip apart. It'll open up with some gentle prying.
-Remove the two screws from either side of the 'U' shaped piece of plastic in front of the receiver. Then slide it forward.
-Remove the four screws that connect the loading tube to the inner body and the four smaller screws and their lock washers that connect the pump handle to the connecting arms.
-Slide the whole barrel assembly forward. You should see the three inner barrels and part of the plastic inner body beneath it. Make sure you account for the plastic end cap that fits on the front of the inner barrels and keeps them steady. It may be stuck in the barrel assembly you removed. Tap it out and put it back on the inner barrels.
-The next part is tricky. You may end up cracking part of the receiver. You have to pull apart the sides of the receiver and slide it forward off of the internals.
I'll try to post that link to the guide if I find it, or maybe take some pics of my own. Good luck bud...
EDIT: Waaa, beat me too it.
Not the guide I was thinking of, but one I haven't seen yet. Nice one!