CONSTANT OFFSET METHOD
FOR ZEROING IR LASERS
Zeroing an IR laser is the same as zeroing an Aimpoint or other optic, in that we are accounting for the drop of the bullet by adjusting point of aim (POA) so that it coincides with point of impact (POI) at a specific distance. Most IR lasers have an additional complication, though- horizontal offset from bore. In order to avoid having the laser pointer blocked by a front sight post, most IR lasers are offset to either the left or right. This creates a lot of confusion...
When we started studying and teaching night vision employment in the early 2000's, lasers were being 'zeroed' using one of two methods- each of which was wrong.
BORESIGHTING IS NOT ZEROING
For a long time, we commonly saw military units, LE agencies, and various individuals boresighting their lasers- but referring to the process as 'zeroing'. They accomplished this using a number of different methods, but the end result was that vertical and horizontal offset were aligned and fixed in relation to the bore. If, for instance, the laser was 1 inch above and 1 inch left of bore, the laser would be aligned so that at some distance (i.e. on a piece of paper at the end of the barrel), the laser remained 1 inch above and 1 inch left. This is obviously NOT a 'zero', since it doesn't account for the drop of the bullet and it is NOT an acceptable way to align a laser on a fighting rifle.
POA=POI AT CLOSE RANGE
We also saw a lot of people adjusting so that POA=POI at close distance (25m or less). This causes gross error in both windage and elevation, since we still haven't properly accounted for bullet drop AND we have created a converging/ diverging angle horizontally that causes a POI shift that is unpredictable and increasingly large over distance. As an example, if our laser is offset 1" right and we zero so that POA=POI at 25m, we will be off to the left by 1" at 50m, 3" at 100m, 6" at 200m, etc. See the diagram below for a visual on this.
'CLOSE ENOUGH' ISN'T GOOD ENOUGH
Both of these methods were 'close enough' to allow them to proliferate through very competent organizations. The problem with 'close enough' when we're talking about zero, though, is that it is only 'good enough'.... until it isn't. It works until we have to make a shot where the enemy is using cover and our error is causing our rounds to impact harmlessly into that cover. Or until the error is causing peripheral hits on a determined fighter, who is able to continue to send rounds our way. Or until there's a hostage situation. So, our position on weapons zero is unequivocally that 'close enough isn't good enough'.
We coined the term 'constant offset method' in the early 2000's to describe what we believe to be the proper way to accomplish zero with an IR laser ON A 25M ZERO RANGE (at the time, we were the lone voice in the wilderness). This was/ is important because, for mostly logistical reasons, most nighttime zeroing was/is done at 25m.
With this method, we account for vertical offset (height above or below bore) exactly as if we were zeroing our day sight. Our objective is to adjust the laser so that POA=POI at our zero distance (i.e. 200m). Doing so accounts for both height above/ below bore AND trajectory, each of which affect our zero in the vertical plane. When we have to shoot at close range (25m), we do ballistic calculations to determine the approximate up/ down shift in point of impact at that distance (i.e. 2.5 inches low at 25m).
We treat horizontal offset as an entirely separate issue. Because there's no significant change in bullet path on the horizontal plane, it's a simple problem with a simple answer- we accept the 0.75 right or left offset of the laser and maintain it at all distances. This gives us a known shift at close range and, more importantly, prevents the very dangerous converging/ diverging laser problem that a short-range POA=POI zero creates.
A final important note. This method produces functionally the same result as POI=POA at extended distance in the horizontal plane. Any time we establish zero on a short range, though, zero should be tested/ confirmed at our actual zero distance (200m) every time we are able.