Re: 223 min barrel length
The limitations are the legalities of what you decide to go with rather than the performance from a shorter barrel. I used to own two Sig 556 pistols with 10" barrels. They performed quite well out to 100 yd...never tried any further. If I hadn't moved back to Kalifornia, I'd still own them. I would not want to be on the wrong end of one of those unless I was <span style="font-style: italic">at least</span> 200-300 yds away. So you could stick with a 16-18" and not worry about any "extra" legality issues, you could go with an SBR setup with something more like the 9" or 10" up to 12" barrel length, or even something similar to the Sig 556 pistol. If all you want is something CQBish, ballistics shouldn't be a huge concern at ranges under 50 -100 yds, the shorter barrel will do the job. As you don't seem to be interested in a gasser, a 16-18" bolt gun would do everything you'd want out to 500 yd or more, with no legal issues above and beyond any normal gun purchase.