;D Nice addon! Here's my review, I'll be as nit-picky as possible:
I noticed something a bit off with the chopper at first glance. I realized it was the demo UH60 model. With that comes a few bugs like a small white patch on the side, the doors looking as if there is no glass in them, and the internal textures flickering when looking through the propellors.
Anyway, to the gun...
The sound is okay, but it is a little too long, so if you fire one shot it will be still sounding off. I would recommend either making the weapon fire in long bursts (may not be realistic, then) or shortening the sound somehow.
The actual model is nice, I would recommend having a spinning barrel, if possible. That would be an excellent touch. Also, you may want to make the ammunition belt go further back so it does not hang outside when the gun is pointing forward. Also, correct me if I am wrong, but is the gun floating? A simple pole going to the ground would be good, or whatever you see best.
The texture is also very nice. If possible (I do not know how textures are applies) could you make the rings around the barrels not have those horizontal lines, and have a texture for the ends of the barrels too?
One non-cosmetic bug is when gunning, the aiming view (what you get when you hit the insert key) does not have the same range of motion of the gun, and so you cannot aim as low as the gun normally can. In addition, the way it is positioned, the player's eye is inside a top part of the gun, so you can see little lines.
On the good side: the bullets disperse nicely, there is no silly explosions where the bullets hit and the choice of firing rates is great. It sure is a hell of a lot more effective than the .50!!! ;D
One last thing. I made a small mission where my chopper is overlooking a small crowd of civilians. When I open fire on them (I am a very bad man) they start flying around! It is fun to see, although I am not sure if this is a bug or not...
GOOD WORK!!! ;D