I have been going thru the 95 page Pic Thread.  In this thread, a couple members said that they had modified their Mossberg's so they can do a "Slug's Only" part, without loosing any shells that are in the mag tube.
What do you need to modify, and how?

Are you sure they aren't talking about 930s or benelli auto loaders? I don't think you can "mod" to do slug changeovers on a 500/590 without losing a shell.

Here's a video of a guy using a 500 keeping the shell in the tube, but it's a pain compared to an 870. I tried it with my wife's 500 and it's awkward as hell.
http://www.youtube.com/watch?v=Oh3YVTgAxJ8&feature=colike

That's similar to the method I use however my Mossberg is set up to make it easier.

To do my slug select method.  First you need to file out a little clearance on the front of the shell lifter.  Don’t worry this has never caused any kind of malfunction, if you removed too much it might though.



Second modification is that you must have an AFG with an enlarged hole in the middle of the triangular area.



With a full tube and a loaded chamber you press the action unlocking lever and crack the action open with the forend



This is where the shell lifter and AFG mods come in.  The AFG mod should give you enough room to get your finger inside the AFG.  The shell lifter mod should give you just enough room to get the tip of your thumb to the tube when the lifter drops.  You press the last shell in the tube forward just enough that it won’t load when the little shell interrupter jumps out of the way.



With your hand in that position pull your index finger toward the thumb which will fully open the action ejecting the shell from the chamber while simultaneously preventing a round from being fed from the tube.



Now reach forward and use your thumb to press a slug out of the two shell holder into your hand. (My hand isn't really in the right postion here to catch the shell because that hand is also holding the gun in place for the picture)



Reach under the gun and toss the shell into the ejection port, then and slam the action closed.

It sounds a little complicated but it is no more difficult than getting a reload into a magwell. I can easily go from being on target, fully loaded with buck shot, to getting a slug down range in less than three seconds from the buzzer on my shot clock.

DTakas,  Thank you so much for your posting this !!
I am going to give this a try.
BTW, your Mossberg looks Mean as Hell.

Can you do a vid of this?  

MAHA

Nice detailed post up above, but I seem to remember it not being so complicated. IIRC it was similar to the 870 way.

Try putting the meat of your finger on the magazine tube just fore of the receiver and pull the forend to that point.  Then press the shell in the mag tube forward and release, then pull the forend the rest of the way.  Does it release a shell?