Aren't the Mossberg 500's 5+1 shotguns? If so, you need to make a plug just a bit shorter than 3 shells, end to end. That would make a 2+1 gun.
I dunno, 'bout 8"??
If you make a plug, be sure to bevel the ends somewhat. If cut off square it's possible for the plug to get tangled in the spring. It happened to me, fortunately the turkey was already on the ground when I went to load another shell and got nothing.