My IR laser is mounted on the top pic rail on my HG. 1.5" under the center of the day scope. Currently it is in the center of the crosshairs at 100 yds. Is this the correct point or should it be 1.3" under the point of impact at 100' to maintain zero?

If it is centered on the crosshairs when on a target at 100 yards, then you're good to go. Your laser will have different impact points than the scope at any other distance, but the difference should be small inside of 100 yards.

Essentially the zen of zeroing any laser, offset or inline, visible or IR, is to align the horizontal axis exactly n-units of measure off of the zero in whatever direction of offset. However, because the vertical component always changes, in that regard we will be ignoring that for now.

So, in the interest of brevity, I will describe it as such. The bullet impacts the target. The laser should be zero'd the same offset distance at the point of impact as the offset distance from the centerline of the bore to the laser aperture. Essentially, you want the laser to be exactly parallel to the bore line. As such your laser will always be some distance offset from the point of impact, and ideally the same distance at any range. Obviously at ranges long enough for spin drift to come into play this no longer applies. This is the ideal. THE LASER WILL NEVER ZERO IN THE HORIZONTAL COMPONENT if aligned correctly.

Remember, the greater the offset, the more drastic the POA/POI difference will be at distances beyond your zero, if you choose to zero at the same POA as your optic and don't take the offset variable into consideration.

That's the horizontal component of the issue.

The vertical component of the issue is essentially moot. Because a bullet is not a laser, the vertical offset from POI must be decided upon by the shooter. Best practice is to either zero at some known distance used as a reference for holdovers (i.e. 100m) or the most common engagement distance. If you're a farmer and are consistently taking 135 yd shots at hogs on your property, it is probably easier to zero your vertical component of offset at 135 yds. Point, click, done. However, if you're taking shots over a wide variety of distances, it is better to zero for a known distance, and use your holdovers based on known bullet drop data.

Remember, horizontal offset means your POA with the laser should be n-distance horizontally from the POI where n is the distance from bore line to laser aperture. Vertical offset needs to be decided upon based on the needs of the individual shooter.

Here are the targets used by the Army to aid in zeroing their lasers. As you can see, they offset based on horizontal distance from bore line to laser, and vertically to coincide with their standard operating procedure for a battle sight zero.


Now, the caveat. Realistically within laser engagement ranges that most people will be using, it won't matter. The offset distance will normally be close enough that doing a true POA/POI zero with the laser will probably not have any serious effect on your shot placement.

If your scope is zeroed at 100 yards and the laser is in the center of the crosshair at 100 yards then they both are zeroed at 100 yards. Since the laser is mounted below the center of the scope it will not carry zero as far as the scope. At say 200 yards your scope may be shooting 2 inches low, and the laser will cause hits lower than that. Maybe three or four inches low. At point blank range it will be opposite. Shots through the scope at ten feet away will be about 2 inches low and shots with the laser will be only about one inch low. Higher mounted optics will let you stay on a given size target at longer ranges but will hit lower at close range.

The whole fixed mechanical offset - offset boresighting target - use the same hold as your day sight is all good and well and sound theory - however, IMHO and IME, in practical terms, it's a lot more complex, and a lot more math, frankly (I'm an English Literature graduate student... needless to say, my mathematical aptitude, even for simple calculations is limited ) than I'm interested in concerning myself with.  

Essentially - if your day optic is "on," if you cowitness your IR laser to your day optic at the furthest practical distance you're capable of, your POA/POI offset will never exceed your mechanical offset inside the distance that you zeroed at.  Moreover, on the far side of your zeroing range, your POA/POI offset will still be within mechanical offset, just in the other direction until twice your zeroing range.  

Meaning that if you zero your laser at 200m, inside of 200m, you just use the same holds with the laser as you would with your day sights, and your POI will never be further off from your POA as the laser itself is from the optic, i.e. less than the frontal profile of your rifle itself until you reach 200m.  Outside of 200m, you'll begin to "drift" out in the other direction, where the laser's trajectory will cross the LOS of your optic, but once again, it won't deviate further than the "mirror image" of your mechanical offset until beyond 400m.  Mind you - you're usually reaching beyond the realistic range of target PID, even with the best Gen. III NV, and beyond the useful range of an IR laser for point targets as opposed to a dedicated magnified NV sight, or CNVD with a magnified day sight.

Moreover - particularly if you have a slaved VIS laser, but even without - this method is easy to do, quickly on the fly, easy to confirm, and easy to adjust if need be.  Just flip on your day sight, turn on the laser, sight in a target like the corner of a building or window at an appropriate range - check to make sure their still co-witnessed, and call it a day, rather than needing specific targets and offset calculation, etc.  

~Augee

Cool video.... but can't you just hurry up and invent a laser that curves just like the ballistic path of the round I'm shooting?  

My shooting is limited to 100 or so yards ordinarily. My IR laser is just left of the bore centerline.

I zeroed the scope in daylight. After dark, I temporarily removed my helmet PVS-14 and placed it behind the scope and moved the IR to the scope dot at 100 yds. Then I looked at the smallest objects I could see about 800 yds away (as far as I can see something I can verify the IR dot on from here) and returned the IR to center (with horizontal IR laser corrections only). A 3X magnifier helps a lot with this.
Re-check apparent zero and test fire at 100 and I was done.

Presto, I am within a few inches vertically inside of 150 yards, and an inch or so left of dead-on horizontally everywhere it counts, and it took 5 minutes of after dark "work".