Barrel length has little to do with accuracy. An accurate barrel is an accurate barrel regardless of length. Length vs thickness can affect barrel harmonics. A shorter, thicker barrel will have less "whip" and is potentially more accurate. Many refer to these barrels as "bull" barrels.
A 16", well rifled barrel that is 2" in diameter will have almost zero "whip". A 20" minimum diameter barrel (pencil barrel) will have decided "whip", but not necessarily inaccurate.
Chrome lined barrels tend to be less accurate than chromed. 5r rifling is renounded for accuracy. For accurate shooting in an AR, look for a Chrome Moly barrel with cut rifling from a good name maker. Even then it depends on the ammo.
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