TFB TV - Myth Busting: Are Longer AR15 Barrels More Accurate?

The qualifier in this is "Range".

The longer barreled rifle will have more velocity at longer ranges, which will make the bullet less susceptible to outside factors. Less wind drift, less wind drag, more accuracy at longer distances. However there is still a series of trade offs. With shorter length, you have something easier to carry, easier to maneuver in tighter corners. However at longer ranges the loss of velocity means you lose effective "Range", and hence accuracy.