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Where to start…
It’s huge and really complicated, especially when compared to the mount it replaced; a metal bar using two pins to hold it in the window and a single pin attaching the gun. Because it is huge it renders the crew door useless for getting in and out of the plane. Being over engineered, the GAU-21 can also be troublesome to trouble shoot. Where as the XM-218, I could almost close a baby in the feed tray cover and it would still shoot. This may have been because most of the guns were new, new guns need to be broken in. The same with any auto/semi-auto firearm.
The size also makes it almost impossible for any crew of shorter stature to properly employ the gun. Unless standing on something, they can’t shoot down. Being in a helicopter, most things you have to suppress are down. Luckily, I’m 6′ 2″.
The biggest issue, ALL CREW, voiced, but the Marine Corps ignored, was the egress issue. In a crash, the old gun and mount sat low enough in the window you could easily escape over them and out the window. If not, one pin and the gun goes out the window. It was often briefed, that in the event of an emergency, go ahead and push the gun out on the way to the ground.
The new gun and mount being huge makes this impossible. If you look at the picture, most of that gun is actually mount. His trigger hand is on the mount. You could try to disassemble it and push it out the window but taking an almost eighty pound gun out of THAT mount can be a real pain in the ass. So, the Marine Corps said, “Go out the ramp.” The problem is, if the cabin is full of cargo (being a cargo helicopter, this happens all the time) getting from the front of the cabin to the rear can be difficult to impossible. Compounded by the fact the cabin may now be twisted metal, and/or the gabillion pound main gear box broke through and is in the cabin, and/or fire.
Because of all these issues, during my last deployment, when we were supposed to switch over from XM to GAU, we left the GAU’s in the box and kept using XM’s.