Apache's New Infrared Targeting System Spots Foes When They Fire

Are enemy muzzle flashes different from friendly ones then?
I smell yet more 'friendly fire' incidents...

That is a very good point.

There is an old saying about "Green stuff coming in, red stuff going out" (well, that is close to the saying), but that distinction is easily clouded.

The reference to red and green are the tracer colors. I am unsure if this is still the way it is, I have not heard the phrase "Green stuff" for a while.

if I helicopter fires at you, i think it instantly becomes unfriendly... then it is a matter of who shot who first...

You are assuming the helicopter is firing at a muzzle flash directed at it.
Imagine you are flying low towards a combat situation at night the combatants positions are poorly defined...the sat nav is telling you 'at the next exit make a U turn and yet your sorry ass out of here'
You pop up ove a ridge there is a sea of muzzleflashes left and right. Which is the enemy?
You apply the USA combat imperative and shoot first ask questions later

I think you just defined "military intelligence"

From my personal experience, if you're shot at in a helicopter you usually don't notice the muzzle flash as the flash suppressor on the weapon makes that hard to see from the air unless you're pretty close. What immediately gets your attention is the (in the case of the NVA/Viet Cong) green BASKETBALL heading your way.

I kid you not, a tracer round, which is usually every fifth round, coming your way at supersonic speeds is HUGE.

I believe AK-47 ammo still has green tracers and I've also seen green tracers from 33mm and 37mm weapons. With those latter two, though, the muzzle flash is very apparent.

I've also read about acoustic sensors that can pinpoint a mortar location and return fire after one or two incoming rounds.

In any case, when it comes to trying to decide whether the shooter is friendly or not, it's usually better to have precise location info than to not have it. It's getting a lot easier to identify the good guys on the digital battlefield, and the process of elimination identifies the bad guys.

Hooker