Niether maker offers chrome-lined rifles.

Understand that these companies are selling overly-expensive target rifles with some questionable "tactical" features as combat rifles.  They aren't.  Combat does not require .5 MOA accuracy, and you would not want a combat rifle with all of the compromises needed to achieve that level of accuracy.

A free-floated chrome-lined Colt or Bushmaster barrel will give you 1 MOA accuracy (with GOOD ammo; don't expect that from 2 MOA M193, or 3 MOA imported surplus).  That's far more accuracy than is needed from a field rifle, and doesn't require a heavy barrel or tight chamber.

-Troy

Your M16A2 is *probably* capable of quite a bit better than that, but NOT with M855 ammo, which is far from a match-accurate load.  But if your barrel has any significant throat erosion, your accuracy will certainly suffer even greater.

But a new M16A2 or M4 should be able to get 3 MOA with M855, and a FF system like a SIR or FF RAS will improve this further without compromising the rifle's reliability or forcing you to carry an extra pound or more of barrel.

IMO, tightening chamber dimensions is NOT the right way to attack the accuracy issue with a combat/defensive rifle.

-Troy