Love talking guns, but the 5.56 was not designed to tumble upon impact to keep it from going through walls. It's designed as light ammunition, so that soldiers and marines could carry more rounds vs the 7.62 which had been a standard mostly. The fact that it's a light round, means it will tumble (like the round that was shot at JFK) and military review sees that as a benefit, because a tumbling round will cause more damage to muscle and organs. The military didn't review and decide on the 5.56, that is based on the .223 Remington Arms round, because it didn't go through walls, I'm not familiar with that being a consideration or question on that round in any literature I've ever read.
I'm not sure how anyone would say a .12 gauge is not a good HD weapon in the right hands. I've got an M4 in 5.56, AR10 in 6.5 Creedmoor, and multiple shotguns. In a HD situation, if you are stopping to put ear protection on....that's interesting is all I have to say. The benefit of a pump....they will run, had many, you can do whatever, rack it and it goes. HD....that's thing 1 to me, if I need it will it go 100%. I'd rather have a handgun personally in close quarters. My m4 is good, I don't leave the suppressor on, so sound is gonna be loud as hell, louder than any .12 guage I've ever shot. It's a great system for HD if you know how to use it, not sure I'd suggest it for the very first firearm. Something with a Surefire that will disorient and intruder with light...is a good idea.
I love my Sigs, and personally I just like my handgun ammo to start with a 4....and my favorite ends with a 5, ha. But my .40 Sig P226 is excellent as well Revolver, frankly, fits the bill, they go 100% (my semi's I've never had a jam, but it's can happen).
So, caring about "loud" I think is a waste, they are all gonna feel loud at 3am. More importantly, they are ALL LOADED, all the time, treat them that way, and only point at what you want to shoot and kill. I don't understand rounds for HD that are meant to wound. If that person is there to kill you, I think it's dangerous to give them a 2nd chance.