by Boikat » Sun Dec 14, 2008 10:20 pm
It may have the gun muzzle flash, but the ground impact effect is from adding FX_firework13 to the smoke system in the .cfg at about 15000 feet in front of the aircraft:
[smokesystem]
smoke.0= 15000, 0.00, 0.00, fx_firework13
smoke.1= -8.00,-0.20, 0.10, fx_hueyL1.fx
It's not a perfect effect, but it works. It also gives an interesting display if you make a run at a tower and replay and view from the tower view. If you aim it just right, it puts on a pretty awsome display that looks like fragmentary ordinance going off.

"I reject your reality, and substitute my own" Adam Savage, Mythbusters