You’re gonna want something that you can shoot accurately and consistently, bar none. You can have the most tricked out AR at the range, but if you cant shoot from anywhere but a ransom rest, you’re gonna be literal dead weight if SHTF. I cant tell you how many Captain Americas I see at the range shooting from these fancy rests and bragging about their quarter sized groupings. If you cant shoot your weapon efficiently from multiple positions or move fluidly through a building with it, you’re better off going in with a wiffle ball bat and some rocks.
Your weapon is only as effective as the body operating it, forget the caliber or barrel size or how many rounds it carries. If you aren’t fit enough to shoot and move effectively, don’t use that weapon. And please do not say “use the 5.56 because that is what the military and police uses and it will be abundant”. Those M4’s are going to either be in the hands of the soldier using it, or on his body. And if you run into a military force carrying an issued M4 carbine with military grade ammunition and have to explain yourself, you’re going to have a bad time. Seriously.
And another thing, if someone isn’t proficient or comfortable using a firearm, then for all that is good and holy DO NOT give them a firearm. In a stressful situation, you’re more than likely going to get yourself shot than keep your loved ones safe. I train with guys I would trust my family’s life with, but even they make mistakes now and then when it comes to muzzle awareness. I would be leaps and bounds more comfortable knowing that my sister and mom are hidden and away from the conflict then having engage in a firefight with someone who is willing to do harm to them. Just my two cents, take it as you will.