Try placing sounds in main Sound folder of FSX instead of CaptainSim/Sound, of course modifying path to correct one in .xml. It worked for me, when I was making sound for CaptainSim Weapons GAU-8 Avanger gun for my IRIS A-10 Thunderbird. For bullet I used CSW Vulcan with tweaked range, weight and 200 burst rate (couldn't figure how to tweak RPM).
About sounds I think YouTube is nothing short of a gold mine in regart to any imaginable military sound. I using about 20 different gun sounds on my FSX planes - all from YouTube.
By the way, when searching: Vulcan, Avanger, GAU-8, Gatlin gun, Phalanx, CIWS - all sound very similar.