Parden my "first post" here going long-winded, but I have been following the board and enjoy reading (and learning). I have a little 20 gauge Mossy tactical for HD and love it, so I thought I should post on my experience with it. I have shot 12 gauge many times, but grew up shooting with my other little 20 mossberg 500 given to me when I was 12 for hunting rabbits and birds, (I have dropped pheasants while the guys with 12's missed a few), so when it came time to pick a gun for the HD "house gun" I went with the Mossberg 50145 20 Gauge.
For HD use, I love the little 50145 with a mounted light. We have been practicing and patterning with all kinds of different rounds to see what they do at different ranges that would be typical in the house, plus shooting some slugs which hit their mark easily at 50 yds. Although slugs at 50 yds does not typically come into play in a HD scenario, but perhaps for a stand off or defending the house situation. But it was sure "eye-opening" to see what the pattern does at different distances.
Things I notice about it and some of my comments are...
(the point in the general direction theory should not apply in HD...A gun needs to be aimed and at close ranges the pattern is tight
You are still responsible for the pellets that miss the target and go beyond to potentially injure others
Try different ammo, there is a difference in pattern spread
Ammo is not as available as in 12 gauge, but there are some good choices
The gun is lighter and very easy to swing on target
Less flash / less noise /less recoil = less disorientation in a closed inside the house situation
Its a lot of fun to shoot and practice with at the range.
my wife (and boys) are not afraid to shoot it and in fact she loves it
I realize 12 gauge has more stopping power, but since when is the equivalent of 2 .44 mags not enough? I read an article on Chuckhawks that made me feel pretty comfortable with my own choice (it references Massab Ayoob recommendations, shown below).
I specifically recommend the 20 gauge for women and recoil-sensitive men who dislike the blast and recoil of the 12 gauge. "Delivering roughly the ballistic force of two .44 Magnum rounds at once," comments the knowledgeable Ayoob, the 20 "delivers 75% of the lead for only 50-60% of the recoil". Many police departments have found their officers shoot much more accurately in realistic training exercises with the lighter-kicking but still potent 20 gauge.
I would add, that it is all about what you feel comfortable with and personally for me that little gun just feels like an extension of me. Most would probably love and feel comfortable with their 12 gauges, but the 20 is not underpowered. (Even though my brother teases me about it needing a pink stock). Real men aren't worried about having a 20 gauge
I guess I am not so concerned with 12 vs 20...the main thing is that you have something and you practice with it. It needs to be stored where you can get to it, but obviously away from where kids can find it. And a security system that beeps to warn you of someone poking around outside the house is a good idea to eliminate the "late night surprise wake up factor" before they ever come in.