This may be heresy, but I sometimes wonder if it could be a bad thing for a civilian to have lots of rounds on board.
Why? Because if 6 rounds or so haven't solved the problem and you can't get to cover to reload your "low capacity" weapon, then you may be dealing with multiple adversaries that have overwhelmed you and/or you are desperately fighting with somebody on the ground. If so, then there's the chance that you could lose your own gun in the brawl.
If this happens to me with a revolver, I'm pretty sure that I will have shot most of my antagonists and teh gun is empty. So they will gain nothing by attempting to get my unloaded gun. But if I'm shooting a hi-cap Glock and get overwhelmed, well, it would be terrible to be shot mutliple times with your own gun using all that extra ammo you brought to the scene.
I have no data on this. I could be totally wrong. I'm sure there are times when it would be wonderful to have extra ammo.
But I sometimes think we worry too much about the rare possibility of needing lots and lots of extra ammo, and we never consider the possibility that there could also be rare circumstances in which lots of ammo might be counterproductive.