The mass of the projectile, multiplied by its impact (terminal) velocity will determine the level of detonation. So, using distance by itself is difficult to determine. Firebird, attached to the bottom of a Clay Pigeon has been tested with number 6 shot at up to 35 yards. The effect will be appreciated far more by shooters and spectators alike if used at a range of 15 – 20 yards. One of our local SWAT teams practices regularly to 600 Yards (and further).
We test each batch of Firebird Targets using everything from a high-velocity Air Rifle with .177 lead pellets, to a .22 LR rimfire rifle with subsonic 40 grain bullets, to a 20-gauge shotgun with 6 to 9 shot size.
Perhaps trying various distances on your own will give you a better answer. We would love to hear what you discover. Please remember the minimum safe distance is 10 meters from the target.