Myth – “You must always go to mechanical zero before zeroing.”
Fact – You’re almost always better off starting with the dope already on the gun.
Here’s what the Army says about Mechanical Zero:
FM 3-22.9, Page 2-4
“Mechanically zeroing the weapon is only necessary when the weapon zero is questionable, the weapon is newly assigned to the unit, or the weapon sights have been serviced.”
Mechanical Zero is nothing more than putting the sights in their centermost setting. This setting has no direct effect on the relationship of the barrel, therefore has no direct relationship to any zero. Unless the sights have been completely replaced and the rifle has never been zeroed, you are better off starting with whatever zero the last shooter used.
Knowing and basing a zero from Mechanical is important only if you plan to “click on” corrections. For example, both SDMs and competitive shooters may dial a True Zero onto their sights based on specific environmental conditions for a given shot. A given engagement or string may require several sight adjustments. Should the shooter forget his dope he can dial back to MZ and return to a No Wind setting, based on his previously recorded data.
For shooter who will only adjust to and use a Point Blank/Battle Sight Zero this isn’t necessary. Hint: A gun owner who can’t explain what “Minute of Angle” is should stick with a BZO and not worry about MZ.