Then it's not accurate at any distance.

Accuracy comes from a chamber and throat that are concentric to the bore, a smooth cylindrical bore of constant diameter with rifling that is uniformly deep and concentric, and a crown that is concentric and normal (90 degrees) to the centerline of the bore.

A chamber that is loose will hurt a little.  A tighter neck will help a little if the chamber warrants that detail.  It's impossible to conclude whether a 5.56X45, .223 Rem, or .223 Wylde chamber will produce the best accuracy without all the rest of the details such as jump to the rifling and the details above.

A little choke in the bore from the throat to the crown won't hurt and is a benefit with lead bullets.  A bore that gets larger as it reaches the crown can be an accuracy disaster if it's more than a couple of ten thousandths of an inch.

Bores that have regions that increase in diameter then squeeze back down might shoot okay, or like crap depending on the details.  In fact all those factors depend on the details, and the definition of accurate changes depending on the shooter's requirements (including cost) and expectations, but "as accurate as feasible" is always good.

First misconception I see above -

Rifling twist rate is determined by the length of the bullet, and the muzzle velocity to a lesser degree.

Longer bullets tend to weigh more, but that is a guideline.  My Contender Carbine with a .224 Rem chamber and its sad 13 5/8 twist will shoot a 52 grain Matchking, but it won't shoot a 50 grain polymer tipped Sierra bullet due to the extra length from the tip.

The rest of the stuff we shoot in .223 Rem jumps larger weight increments, 55 to 62 or 69 grains, 69 to 73 or 77 grains, then to long high BC 80 grain bullets, so the perception is that rifling twist rate follows bullet weight, but that is not true, it's bullet length.  Longer bullets, faster rifling twist required.

I shoot 1:7 twist barrels other than a Colt and the Contender.  The argument is made that very light (short!) varmint bullets will spin so fast that they blow up - none I have fired down to 35 grains blew up, but I haven't fired every varmint bullet.

I you don't believe you'll ever shoot an 80 grain bullet, then a 8 twist barrel will probably serve well.

This... I have plenty of 1n7 5.56 chambered barrels that shoot known to be good ( accurate ) ammo.

And the quality ( consistently ) of the barrel manufacturer matters much more then what the  "spec sheet" says.

If TRIARC would ever produce more 14.5" and 16" barrels, I would suggest one of those... or a Criterion CORE.  

IMHO, spending more for a quality barrel, is a far better investment then spending that same money on ammo, trying to get a lesser barrel brand shoot to your expectations.

The cost of a great barrel is less than the cost of ammunition over its life.