@Tattered,
These two quote triggered some funny memories--
I agree that 3 1/2" 12 ga may be overkill, but either the 2 3/4" round should be fine indoors or outdoors with #4 buck.
I traded my 2 3/4" 12ga shooter to get a brand new 3 1/2" 12ga turkey gun. Except for practice, I have fired it five times. Got five turkeys with the thing. Love the beast, but it has a recoil like a .50 cal.
Peace of mind is a good thing. I assembled my own alarm systems from components and found chips that bark like dogs, but not one that sounded like a 12 gauge chambering a round. I think that might be worth its weigh in gold, especially if all the lights come on at the same time.
Back in the Olden Days, one of my buds got some leave time. After a coupla days, he got pretty oiled at the local boozer, and decided to play a little trick on his wife.
When he got home, he snuck around the back to the ground-floor bedroom window, and began to silently lift it up. He got it pretty well opened... and he heard his 12ga being racked back in the darkness of the bedroom.
He hit the ground with his hands over his head, screaming: "Honey! It's ME! Don't shoot!"
When he told us that story, I laughed so hard, tears ran down my leg.