14.5" and 16" barrels seem to be the most popular, because they're smaller, lighter, and more maneuverable, but a 20" barrel will work just fine. Ditto the red dot sight recommendation. They're the fastest at target acquisition, and I've used a Aimpoint Comp on steel silhouettes out to 400 yards with a 14.5" M4 barrel. Learn the holdover for your rifle/ammo combination and shooting long range will be a cinch. A muzzle brake reduces muzzle rise, helps you get back on target faster, potentially saving you fractions of a second, and every second counts. You might want to consider a competition single stage trigger too. I use a JP single stage. It's crisp and light and the reset is short. HTH