That's the problem, though. Finding that right tube is easy. Finding the nut might not be. I know it's out there, I haven't seen one on sale in a while, though and it's extra gold for not much reason. Just get the right tube
Found one on Ebay....you see the cost. You could probably find it cheaper, just don't buy it at all if you don't have to.http://www.ebay.com/itm/Mossberg-500-Action-Slide-Tube-Forend-Adapter-Extended-Nut-12-Gauge-NEW-/191533511426?hash=item2c98497302:g:ecwAAOSwBLlU93G3
Pretty much all of the "tactical" forarms are for the 7 5/8" tube, though. It's still the standard. The shorter tube is mostly on the current wood forarm guns. Maybe ONLY on the sporting wood guns. Not sure anymore. Been a few years since I bought one new off the shelf. I know of at least three aftermarket parts that are adjustable, so to speak, though. ATI has one that's sort of AR type that has a cut off line where you cut it down to fit a 6 1/2", the TacStar Thompson type vert front grip has a cut off ring, the ATI "universal" has adapter thingies etc. By comparison, I can't really name a single aftermarket part that's solely for the shorter tube. Just go for the longer tube and you can't really go wrong. Most folks end up with two or more stock sets inside a year anyway. You will, too.
This is the Tacstar Thompson type. You see the cut off line I mean.
There's a couple of diff MagPul forearm installation vids on Youtube if you want to watch them. You can see the longer tube in both, I think.
But forearms, as they go, you can't really go wrong. It's going to work out for you, it just depends on how many extra days it takes is all. You'll get great experience in switching yours. Mossy's are really user friendly when it comes to mods. there's only two parts that I suggest you stay away from....#1 is the Blackhawk/Knoxx forearm and #2 is the Speedfeed "bird's head" grip. The Blackhawk forearm was badly molded with an actual curve to it like a banana....impossible to fit. The second gen supposedly fixed it. It didn't. Third gen, I haven't held in my hands so I don't know. The speedfeed grip, contrary to what the dealers would tell you, was for the Rem 870, not the Mossy. The Speedfeed packaging actually said Mossberg....the PART however, had Rem 870 molded into it. It overhung the Mossy in places and didn't fit flush in other places. Speedfeed refunded all sales on that one and dropped the part at that time.