Are you sure that mag tube isn't 8 plus one in the chamber instead of 9+1?
I think the Mossberg 500 mag tubes are 5 or 8. I've got a 20" 500 with the long mag tube (holds 8) and a side saddle (6) for a total of 15 shells on the gun when one is chambered and you top off the tube.
I put it up in the WTT section of the site because I actually prefer the weight/balance of the 18.5" with the 5-shell tube.
I couldn't imagine trying to move around my house with that load trying to defend it. Glad my safe room/ reloading area in right across the hall from my bedroom. If I can't get in by the time the me and the wife's scatter guns and 40's are empty it's time to hang it up.