My first experience with the M16 was during qualification at boot camp in 1976. I was slung up in prone, not sure of what to expect when I fired the rifle. When I squeezed off the first round, all I heard was a pronounced sproing...

I looked at the buttstock then back at the Gunners Mate who was coaching me. The GM must have recognized the look on my face. He just said, "Yep, thats what they sound like".