I read the Ammunition Made Hard, and it teaches you how to loose 2 bullets when shooting twice on the same unit. I was just wondering.. is there any possible way to play a gunfire sound twice when shooting a unit that needs to get shot twice?

I don't know any real solution to your problem but try to time the intervals between shots?

Hey, just try the vHP since many of the Ammunition systems are for 1 hit killed units:
vHP is about to use zerg burrowable unit under a certain unit to permit the triggers to "detect" it HP... so each time it decreases, you can play the sound of the shot

You can play the sound twice when the unit dies, but not 1 sound when you fire the first shot, and a second sound when you kill it, because there's no way to detect a unit firing (without EUD's).
So yes, I'd recommend vHP too.

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Hm but you could make a system that 'estimates' the first shot.

1st tier: simply make a location as large as the units' horzizontal/vertical (aka not diagonal) fire arc.
When an enemy comes into range it's likely that your unit begins shooting. => play wav

2nd tier: This will fail when the enemy comes not directly from above, below, left or right. (Shot sound too early)
To counter that make a large mobile grid around your unit and detect the direction. If it comes in from diagonal direction use a smaller location for the firing arc.

3rd tier: This will still fail when your unit moves instead of shooting. So refine the system by making a move detection with a 1x1 pixel location: Center the 1x1 location on your unit. Let a dc count to lets say 6 and if it hasn't left the 1x1 location yet, it hasn't moved and you can play the sound.

4th tier: This system may produce inaccurate results when multiple enemies are closing in. I have no general method to present which would counter that. It depends on your map, too. So if you decide to make a 'shot estimation system' maybe we can work out 4th tier later.

Wait.. sorry.. explain to me whwat vHP was again.

Basically you make the main unit invincible, constantly move like 20 zerglings under the unit, each representing 5% hp. When all lings died kill the main unit.

Tutorial on vHP
Another Tutorial on vHP