The term "Zero in" probably has other meanings, but in artillery or firearm disciplines it has to do with adjusting the aiming system to hit a particular target. This can be done either by "walking the round" (reading where rounds hit and making adjustments), "holdover" (aiming the sight above/below target based on prior experience), or by various computations (it's all Newtonian physics)
For instance, a call to set a 200 meter zero would dictate you set your scope or sighting system to the settings (declination, windage, elevation, etc) which would (through prior knowledge/practice/computation) guide the given munition to a particular spot 200 meters away.
Other trivia (easy) - where did the term "Whole 9 yards" come from? 27' munitions belts.
"I said, now, I said ,pay attention boy!"
The cure for anything is salt water - sweat, tears, or the sea
Isak Dinesen
If a man is to be obsessed by something.... I suppose a boat is as good as anything... perhaps a bit better than most.
E. B. White
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