[RunningOnMT]

My totally uneducated guess is that a 20" barrel might be just a tad more accurate than a 16" due to increased velocity. Increased velocity = less bullet drop over longer ranges because the faster bullet hits the target sooner. But that difference in velocity and it's corresponding effect on the POI would be so slight it would likely be imperceptible.

But I said that was a guess. For a real answer JLA is the man to tackle this question. He has spent many hours scientifically studying such things, and has corrected several misconceptions of mine regarding barrel/bullet mechanics.