DouglasW
I'm on your side, but I'm playing devil's advocate...
Even with a standard 2 round extension (that complies), the factory specs say the mag capacity for a SPX is 7 and, I'm guessing, that's what the authorities would go buy.
tomgun"inside my mossberg 500 was a dowel to limit the amount of ammo.
simply remove it or trim to your desired amount."
A magazine plug (limiter) is acceptable in the enforcement of hunting regulations. However, since it's a temporary item, I doubt the anti "assault weapons" folks would accept it. Do you remember, during the 10-round limit era, high-capacity box mags that were "plugged" to 10-rounds had to be welded closed so you couldn't remove the plug?