View Full Version : battlesight zero; axis-over-bore

May 27, 2012, 08:18 PM
I've got an M1A Scout set up in a Sage stock and I'm running a Comp M4 on a Larue mount. This puts the sight axis a little over 2 inches above the bore.

I want to set up a battlesight zero by zeroing on a 25 yard target with the POI 1.75 inches above the POA. Does the sight axis-over-bore come into play and if so, how do I adjust?

May 27, 2012, 09:24 PM
Here's a sight that will help you out:


If sighted in at 25 yds, shooting 165 Nosler BT at 2600fps, you will zero at 25 yds, keep rising, hit 100 yds about 4" high, and then rezero at about 265 yds.

This is just a general estimate, real world will be different, so try in at the range.

If sighted 1.75" high at 25 yds, then add the 1.75" for the short distances, but then it is going to drop even more at longer distances. I would recommend a longer sight in distance.

May 28, 2012, 08:08 AM
I'm not sure I understand the BSZ and 1.75" high at 25 yards.

Assuming the present army's BSZ of 300 yards, that would put you .8 " high at 25 yards if your sights are 1.5 above the bore.

You said your's is a little over 2" so at 2.25 you would be .11 high at 25 yards

This is using the standard 168 @ 2550 fps out of an M14/M1A.

If you sight in 1.75 " high at 25 yards with this ammo, your is going to be 471 Yards.

The idea of a BSZ is where you can hit a certain target between the muzzle and a given range based on the size of the target.

Lets assume the E-Silhouette target which is 40 inches high. You want your rifle sighted in at the center of the target and not shoot over or under. Or the path of your bullet not being 9.5 high, or 9.5 low.

For the M852 type ammo that would be about 325 yards, the Army rounded that off to 300. With a 300 yard zero you're never over and don't drop down until about 375. You can take a head shot and you're good to about 425.

I just have a hard time trying to figure out why you want to be 1.75 high at 25 yards.

Maybe I'm missing something.

May 28, 2012, 08:56 PM
Kraig: What you describe with respect to a battlesight zero is exactly what I'm after. The POI over POA I used came from the article included by Springfield Armory in the box with a new M1A. It originally appeared in American Rifleman in 1997. It suggests that for a 300 yard battlesight zero to set the 25 yard zero at POI above POA at 1 11/16" (I rounded that to 1.75"). This seems quite a discrepancy, .11" versus 1.75", and I am at a loss to explain it.

Nonetheless, I am indeed after a BSZ that is on at 300-325 yards. I'm using XM80 (147 gn) Lake City ammo. I'm not sure of the MV of the XM80. I know this is second tier (or lower) ammo and the only ballistic data I could find indicated an average MV of 2731; I don't have any idea about the BC.

With a 2.5 sight height, JBM has it right at zero again at a little over 325 yards.

My real question is how to do I compensate for the excessive sight height. Do I zero the optical sight at the bullseye at 25 yards and look for a POI either .11 or 1.75 above?

Let me also say I am more that ready to defer to your substantial experience with this weapon system. I cannot explain the discrepancy between your .11" figure and the articles 1.75" figure.

May 28, 2012, 09:59 PM

With these numbers your POI would be .4" below POA at 25yds.

EDIT: To achieve 1.75" POI above POA, at 25yd, you are looking at a 525yd zero.

May 29, 2012, 08:10 AM
I have always found printed ballistic information to be an excellent theoretical starting point. But there are to many variables[light,humidity,atmospheric pressure,equipment,shot execution,etc] to conclude that the numbers are absolute. Recorded individual results based on actual firing for future reference ,aka data book, is the only true measure. In short ,set up the rifle with a best guess of the dope and shoot to prove the numbers for that particular gun.

May 29, 2012, 08:25 AM
I was never a big fan of sighting in at 25 yards and "hoping" you're on.

If you want a 300 yard zero, then sight your gun in at 300 yards.

25 yards will get you on paper after installing a scope or something, but its no place to sight in a rifle for longer distance.

May 29, 2012, 10:38 AM
Okay, so now I'm really confused. I checked FM 23-8 (1974) and the BZ0 procedure is to use the 25-meter target and aim at the base of the rectangle (the barn door) and when the weapon is BZ0'd the POI will be 4.6 cm above the POA (using M80 NATO ball) - that's 1.81 inches above the POA for a 250 meter BZ0. That's consistent with what appeared in the article I referenced.

The article was "Zeroing the M1 and M1A Service Rifles" by Scott A. Duff & John M. Miller, American Rifleman, February 1997.

The instructions for a BZ0 are as follows:

"To attain a battle zero, fire the rifle at a small aiming point set at 25 yds. Adjust the elevation knob so the shot group is centered 1-1/4" above the point of aim for .30-'06 Sprg. and 1-13/16" for 7.62x51mm. This setting should allow hits on target from 0 to 400 yards."

Then I ran the M80 data on the JBM calculators for a zero distance of 275 yards (approximately 250 meters) and a sight-over-bore of 2.5". It more or less agreed with Mr. Awesome (-.5" at 25 yards to be on at 275).

My goal is to zero the Comp optic to be able to hit a 20" x 20" rectangle out to about 400 yards. Now I realize with that size target and at those ranges the +/- 2 to 2.25 inches won't make a huge difference. But I would like to understand why the ballistic charts and the "book" values are so far off. I suspect I'm missing something basic here. A little more research suggested that conventional wisdom was that a BZ0 could be attained with the on-at-25, on-at-250 formula.

May 29, 2012, 05:10 PM
The BSZ trick only works reliably when you have a bunch of people shooting the same ammo with the same sights and don't want to bother getting range time somewhere that has pits out to 300yds because it's easier to find a 25yd pistol range.

Once you change your sight height or barrel length (changing both sight radius and muzzle velocity) or ammo you are on your own.

Think about it, if your line of sight is higher, then at 25yds you'll have to have the barrel angled up higher to hit the same spot as you would with a lower line of sight. That means the bullet is going to be way higher at 300 than you expected. Also, if the bullet is going slower out of your shorter barrel, than it will come down a little sooner and hit lower at 300. The "standard" charts pick some "standard/random" sight height. Any worthwhile ballistics program will allow you to put in sight height, as the Hornady web site does. Also remember that unless you are actually measuring your muzzle velocity, then you can be waaaaay off from what it says on the box and your numbers will be off accordingly.

The real trick you want to do is to find your zeroes at 100yd, 200yd, and 300yd, pick a sight setting that keeps you more-or-less centered, and then see where you hit at 25yds. Then at least you can check your zero at 25yds if you need to. After that, if you ever need to supply a few hundred thousand people with your same rifle, ammo, and setup, you can save them a ton of time and energy setting up their rifles with your new BSZ.


May 29, 2012, 06:14 PM
I would like to understand why the ballistic charts and the "book" values are so far off

Because sight height matters, particularly for this.

I am guessing the instructions in the book are for the iron sights? What is the sight height of the irons, about 3/4 of an inch? With your optics, your bullet is starting about 2" lower (relative to L.O.S.) than with irons.

Go back to the JBM calculator. Check the box that says "Zero at Max. Point Blank Range" Go to the "Vital Zone Radius" and put in "10" (the radius of your 20" plate).

May 29, 2012, 07:53 PM
Concur with Kraigway Get it on paper at 25 yards and then verify at 300 yards.

May 29, 2012, 08:44 PM
Thanks all, it has been a learning experience. I will, in fact, get it on at 25 and then zero at 300. And I did get my original question answered, thanks to emcon5 - it is the sight axis-over-bore that makes the difference.

Don't know if this optical sight thing will actually work out like I intend (the same as an iron sight battlesight zero), but its going to be fun experimenting.

May 29, 2012, 09:39 PM
Keep in mind, with any ballistic program, they are only as good as the info you put in. The biggies that have helped me get good results are sight height, and altitude/baro pressure. If like me, your home range is over 4000' and you run your numbers at sea level, don't be surprised if what you get out isn't all that useful.

And always do a sanity/reality check at the range. What the ballistics programs tell you may be pretty good, or it may not......