Mine comes from a nick-name from way back...
I had an opportunity to cross train with the U.S. Army's Sniper school for a two day stint during the Vietnam era (1969). We were provided with the new M21 which was a converted M14 and modified to use the (newer) 7.62 ammo. The first day was all classroom and theory, ballistics, trajectory and night vision - a very comprehensive 8 hour class to say the least.
The following day we were allowed a clip with 7 rounds and fired at a target at 700 yards from a laying, prone and standing position. I hit the bulls-eye near perfect 7 times. I have to say the weapon was dialed in beautifully, was comfortable and the environment was perfect for qualifying. And being an invited guest from the USAF to participate with the US Army was a great opportunity to understand how the two services interacted in S.E Asia.
On the flight line and ready to hop the aircraft back to our base...one of the instructors (Army Captain) slapped me on the back and asked me my first name. When I told him, he chuckled and said "Good luck there Mark Triple Seven". I took a heck of a raze during the flight back...and even more when the gossip started back at the AFB. The name stuck for the rest of my hitch with the USAF....and still, 40+ years later, I use that nickname.