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Some people would say a downside is that the mags are only 10 rounds (although the KelTec can take pre-ban Glock mags, IIRC), but to me that doesn't matter for a home defense system. I would use a high capacity mag in a home defense situation, not even a legal one. My thinking is that especially in MA if you shoot someone even in your own home you are going to be starting from a place where everyone assumes you are wrong, law enforcement included. Most likely they will be looking over everything about the weapons system you used and I'm not giving them anything to work with - I'm not leaving any chance that they can talk about me lying in wait with a high capacity magazine, etc. 10 rounds should be plenty.
I also don't carry extra mags because I will never have a mag-related malfunction. Maybe I should carry a revolver instead; who needs more than 6 rounds, anyway? I mean, revolvers never fail, right?