Zeroing any firearm is a simple process that seems to evade far too many gun owners. The idea is to bring the line of sight (as viewed through the sighting system) in coincidence with the line of bore as to make best use of the trajectory.
A skilled rifleman with a decent rifle and sights should be capable of establishing a working zero after firing only one or two rounds and does NOT need a bench rest.
It is simple, that is, assuming you possess the following things:
- Enough marksmanship skill to accurately fire and call a good shot.
- Enough math skill to use angular deviation (minutes, mils) and make accurate adjustments.
- Understanding on how the rifle and sights in question work.
- Enough ballistics knowledge to understand the trajectory of the ammunition used
- Enough marksmanship skill to accurately fire and call a good shot (so important, I'll list it twice.)
Sadly, most gun owners, even those military trained, have none of the above. I'll give you an example of how this is supposed to work.
The rifle was an unfired M1A. The owner brought it out to our KD (Known Distance) range with a box of M80 ball ammunition. He wanted to shoot it, but was concerned he wouldn't be able to even hit the six foot target frame, much less the E-Type silhouette on the board, as we were shooting on the 500 yard line.
I mechanically centered the sights for windage and bottomed them out for elevation. I then came up ten minutes (M14/M1A rear sights are normally 8-12 minutes up for 200 yards) and added 11 more minutes (3+4+4) as a come-up to go from 200 to 500 yards. There was a gentle breeze quartering from my left rear at around five miles per hour so I hedged my bet with two minutes left.
After adjusting the web sling to give a tight loop, double checking my natural point of aim and dry firing two good "shots" I fired my first live round. Much to my own amazement, being unused to M1A's, the shot broke clean and I was forced to call it as good. The target came back up out of the pit with a five-inch disk online with the waist and out in the white by about one disk diameter.
I put three minutes left and two minutes up on the sight, rechecked my NPA and the wind, and fired another shot. The target came back up disked in the chest just to the left of the sternum. I handed the rifle back to its owner and he was able to put his first shot on the silhouette. One click on the sights and they we all in the chest.
This doesn't take the box(es) of ammo gun owners and hunters seem to expend during sight-in day and it only takes a few minutes. If the shooter isn't ready to fire a group for confirmation after a few shots something is broken, be it equipment or skill.