I don't know why, I always associated it with the sound of some kind of crazy high-speed Zipper. My fav though was we were in one firefight for 45 min, I got to go through about 600 rounds of 50 cal, we launched Hellfires from Chiawas, they did a few gun runs, and our Mk 19 gunner was lobbing rounds through the window of a mosque from where we were being shot at. When we left at least one building in the village was on fire. I found out later when we were re-arming back at the FOB one of my ammo cans had a round through it. So either the round landed a foot in front of me and I never noticed, or it landed behind me and I never noticed. Bad ass gunfight though.