The FBI Says This Is the Best AR-15 Barrel Length

What is the best AR-15 barrel length—and are ultra-short 5.56 rifles actually useless?

James visits the FBI’s Ballistic Research Facility to speak with veteran FBI ballistician Robert about AR-15 barrel length, projectile weight, velocity, twist rate, fragmentation, expansion, and terminal performance.

Robert explains why muzzle energy alone tells you very little, why the FBI prefers expanding projectiles over traditional fragmenting ammunition, and how every bullet has an optimal performance window. Depending on the ammunition and intended distance, a longer barrel can even drive a projectile too fast and produce worse terminal performance at close range.

We also get direct answers on the practical differences between 7.5-, 11.5-, 14.5-, 16-, 18-, and 20-inch AR-15 barrels. For law enforcement and home defense, Robert identifies an 11.5-inch barrel with a 1:7 twist and a properly selected 64- to 75-grain bonded projectile as the practical sweet spot. He also explains why 7.5-inch 5.56 rifles are not necessarily worthless—provided their ammunition and engagement distance are properly matched.

This is not internet theory. It is a detailed discussion of actual projectile testing, terminal-ballistics standards, and the variables that determine whether a 5.56 round performs as designed.