We were instructed to count the seconds between the “crack” and the “thump” to estimate the distance to the person who had shot at us – about one second for every 300 meters. If fired at by a machine gun, we began counting from the last crack until the last thump.
The sound of the bullet cracking overhead won't tell you anything about the direction from which it was fired. You listen for the thump and point in that direction. That's where you find the shooter.
It takes discipline to use “Crack & Thump” well. The first crack starts the adrenalin flowing quickly followed by the exhilaration that comes when you realize that the bullet missed you. It also isn't very helpful in a fire fight when multiple weapons are firing simultaneously. This technique is most effective when searching for a sniper.
I'm not talking about an “offensive” sniper – one shot, one kill. You've seen them on television and in the movies. They wear a ghillie suit to blend into the terrain. They use high-powered, long range rifles with telescopic sights and silencers to suppress the “thump.” They also use special “loads” with smokeless powder and special muzzles to suppress the flash. They infiltrate enemy territory and assassinate key personnel.
We practiced spotting snipers in many different situations and times of day. However, I disrupted one class with the hiccups. The aggressors were well hidden and we were struggling to identify them one evening. Dinner hadn't agreed with me and I let rip with a belch that could be heard over the entire training area. We were then able to spot them easily. They were giggling uncontrollably. It's a technique that I was loathe to apply in Vietnam.