I bought a 9c for home defense and am by no means an expert shot. I hold a pretty tight group at 7 1/2 and 10 yds but not quite as well at 15 yds even though I am getting better. Just wanted to know it these ranges are adequate for home defense. I would like a little tighter group at 15 yds but I figure that will come with time and practice. Is this about the effective range for a short barrel gun?

Depends on how long the halls are in your home or how far across your largest room is. (Bad grammar, I know) I personally have a 12Ga shotgun designated for HD. I doubt I would ever need to shoot beyond 10 yards inside my home. I have several handguns for car and concealed carry. Anything past 15 yards outside my home I would consider running or driving away.

Forget about group size. Get human silhouette targets and practice shooting center mass. Measure the most likely distances in your home that you could expect to have to deal with. The heart/lung area is probably bigger than the groups you are concerned about. If you are not CCing it, get a full size mag or two for home duty.

I have 4 12rd mags. Do they make a larger one foe the 9c?

I'm a fan of tritium sites. Once you've trained enough to pull it up accurately and quickly, get sites that can be seen in low light.
I prefer a short 12 gauge for HD too.

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Piece Keeper

Full size 17 rd mags fit the compact. They stick out the bottom of the grip a little but they work fine. X-grip makes a sleeve that fits over the full size mag to make it look and feel better.