A question about sniper rounds?

Dont know about the .408 but the Barret .50 cal is a sub MOA capable weapon. And yes, a black tipped Armor piercing round can indeed take down a helicopter. In fact, you dont even need a .50 for that or even a armor piercing. The most vulnerable area to hit would be just above the pilots head where the bullet can damage the gear box, the rotor head, FOD (foreign object damage) one of the engines, take out fuel lines, take out the digital electronic control units, the fuel metering units, pierce one of the exhaust ducts allowing for heat build up ect ect ect.. All of witch would cause catastrophic damage.