In essence, a 5.56 round (M193 or M855) will retain much less energy after passing through a sheetrock wall, and will penetrate many fewer sheets of drywall than a 9mm, let alone 00 or 000 buckshot or a slug. At the same time, as long as the projectile impacts with its (human) target at a velocity greater than 2700 fps, the bullet will fragment at the cannelure (sic) and tumble through tissue creating a much more dramatic permanent wound cavity than shotgun or pistol caliber ammunition.
So, its really the best of both worlds - less risk to neighbors and bystanders (overpenetration), and more reliably lethal to the target. That is certainly part of the reason that LE agencies are moving towards 5.56 caliber weapons over subguns.
Can't comment on the Rugers. Bill's advocacy of limited capacity magazines in the era leading up to the 1994 AW ban is enough to keep me from purchasing his products. ARs are proven in the field, have parts and accessories available from everywhere, and are typically more accurate than mini-14s, from what I've read.
Note that the properties of the ammunition listed above has been documented by a number of organizations, including Fackler & company in the Wound Ballistics Journal. It applies to military-grade M193 (55gr) and M855 (62gr w/penetrator) only, rather than applying generally to all 5.56 out there. Good luck..
Yeah, I wouldn't mind having an AR. I have been buying the poor man's AR with the Mini 14s I suppose. I can buy more guns and ammo that way.
Someone else mentioned the 55 grain round as being best. I didn't know that, and I appreciate that info. I have quite a few rounds stored away. I think it is all the 55 grain stuff.