I later learned it's part of their routine in-flight check list, but when you aren't aware of that, it can be a bit distressing.
Few years back, DOD had me go to a FOB while in AFGH. The trip was on a full Chinook. About 30 minutes into the flight the two side gunners opened up with their .50 cal. I was seated in the front so I was unable to see if the guy laying in the back opened up with his .50 cal. They dropped me off while later with engines running. Never found out why they fired.. Figured it was just practice...
There are 1001 ways to get hardcore grunts to puke in their steel pot! I found that in the first few seconds after liftoff was the optimum time to ply those ways. It was all in good fun, for the Crew anyway.