Found this while going through some old assets of mine, had used it by mistake instead of an effect attached to an emitterordnance.

MuzzleFlashEffect = "whatever your effect name is"

Pretty useless for infantry, as the effect appears at hp_fire (ie the head...weird look).

But...you can use it for vehicles.
For many you'll have to offset the effect a bit with PE, but it does work. Examples (yes the effects are pretty bleh and the AAT main cannon one is offset too much)....

Presumably you can also use it for turrets, although I haven't tried that yet.

Couldn't you offset it for infantry as well?