AR-15 Range Report (with pictures): USMC 25m Zeroing Procedure

[Molon]

I recently got a copy of the M4 zeroing target, in addition to the A2 target I already have. I know this question has been asked before, but I've never heard a good, definitive answer. Why does the M4 zeroing target say to put the rear sight at bottom setting (i.e. 8/3 or 6/3), but the A2 target says to put the rear sight at 'z' setting?

To my understanding, the only significant difference between zeroing the M4 & the A2 is the spacing of the grid squares on the target to account for the increased sensitivity of sight adjustments on the M4 due to shorter barrel length. 25 meters does not fall at first crossover for either of the rifles, and therefore I would expect the M4 zeroing procedure to require the rear sight to be set at 'z' just as in the case of the A2. Difference in muzzle velocity between 20" & shorter barrels does not yield an answer.

Anyone know what the deal is with this?

JH

The difference in muzzle velocity is the primary reason for the difference in the rear sight setting during zeroing. If the rear sight of the 20” barreled M16A2 with a muzzle velocity of approximately 3100 fps firing M855 is set on 8/3 (6/3 for removable carry handle sights on the A4) while zeroing for point of aim equals point of impact at 25 meters, the second crossing will be well beyond 300 meters. (Hence the reason for zeroing on the “Z”setting.)

The 14.5" barreled M4 firing M855 has a muzzle velocity of approximately 2900 fps (200 fps less than the A2). With the rear sight of the M4 carry handle sight set at 6/3 and zeroed for point of aim equals point of impact at 25 meters, the second crossing will be near 300 meters. (The 25 meter field expedient zeroing scheme is not exact.)

Also, the sights on the M4 have a greater change in point of impact with each click due to the shorter sight radius, not because of the shorter barrel.

[coldblue]

The Army used a 25m zero range simply because it has 25m ranges almost everywhere. Prior to the M16, the M14 used a 25M/1000inch range to zero, and prior to that the M1 Garand late in it's life used the 1000inch range as well. Because Army posts already had the 1000inch/25m ranges for many decades, it was basically a cost measure to use the standard 25m range to zero.

As far as not using the first crossover point, it wasn't a big deal to the Army. It's a trajectory, so as long as you have the bullets impact in the right spot it will be fine. That spot doesn't have to be the point of aim. Originally we used to zero the M16A1 with the short range sight and adjusted the sights so the point of impact of the group was 2.4CM below the point of aim. It wasn't until some bright guy got the idea that you could just use the long range setting on the rear sight, which would bring up the POI to conicide with the POA and zero dead-on at 25m, then flip back to the short range sight and you'd be set for 250M. Alot less confusion with that method, especially with people who don't understand any ballistics, but for a long time we didn't bother with the point of impact and point of aim being the same. With the M14, you zeroed so the bullets struck 4.6CM above the point of aim.

So for a very long time, we never zeroed at the first crossover, and technically there's no reason to do so as long as you know where the bullets will impact at 25 meteres. As long as they hit in the right place, that place doesn't need to be the same point you're aiming at.

It does aid in reducing confusion with people that don't really know much about ballistics, and that's the vast majority of the Army. Not to say they can't shoot, but the educational base on ballistics, etc. just isn't there. So the Army tries to keep it simple and use a POI and POA that coincide, KISS and all.

When we got the A2, zeroing was the same as what your article outlines. It wasn't referred to as the "Marine Corps" this or "Army" that, and digging in my old FMs, there is no reference to a service branch in connection to the zero procedure. I can only assume that designation was made later.

Another element of the 25M zeroing history is that the Army's BZO in those era's (prior to the M16A2) was 250 meters and they were going metric; and 2.5cm was 1/10th the same "click" distance at 250 meters. The old 25m targets were scribed-out in 2.5cm grid lines. Then the Soldier would go qualify on a pop-up target range with targets at 250, etc.
The Marine Corps spin on the 300M BZO comes from our (USMC) rifle improvement program in 1980-1983 that I led. We had a 300M zero BZO doctrine in those days and built that into the A2 sight. Then through a series of graphic live fire demo's convinced Fort Benning that 300m was what they wanted as well. So during discussions, the term "USMC Zero" came into the discussion. That is most likely why you hear that reference today.
I have discussed this many times in the past here and on ar15.com. If you are really interested you might search out some of coldblue's former postings.

[Molon]

Another element of the 25M zeroing history is that the Army's BZO in those era's (prior to the M16A2) was 250 meters and they were going metric; and 2.5cm was 1/10th the same "click" distance at 250 meters. The old 25m targets were scribed-out in 2.5cm grid lines. Then the Soldier would go qualify on a pop-up target range with targets at 250, etc.
The Marine Corps spin on the 300M BZO comes from our (USMC) rifle improvement program in 1980-1983 that I led. We had a 300M zero BZO doctrine in those days and built that into the A2 sight. Then through a series of graphic live fire demo's convinced Fort Benning that 300m was what they wanted as well. So during discussions, the term "USMC Zero" came into the discussion. That is most likely why you hear that reference today.
I have discussed this many times in the past here and on ar15.com. If you are really interested you might search out some of coldblue's former postings.

I always enjoy reading your historical aspects of a topic. Thanks for posting.

Molon

[Smuckatelli]

0-2

There seems to be some confusion around here as to the function of the “0-2” large aperture of the A2 rear sight. The field manual for M16A1 and M16A2 Rifle Marksmanship states, “The larger aperture, marked 0-2, is used for moving target engagement and during limited visibility.

I think the person that revised the field manual probably was just doing a 'check the box' to get the publication to press.

They should have been more specific, the A1 does have a long range sight, the apeture is the same size. When we would shoot further than 300 yards we would flip the 'long range' sights up.

The A2 does not have a long range sight, it has a limited visability site that is manipulated the same way that the long range sight on the A1 was, I think this has lead to a lot of confusion.

[coldblue]

I think the person that revised the field manual probably was just doing a 'check the box' to get the publication to press.

They should have been more specific, the A1 does have a long range sight, the apeture is the same size. When we would shoot further than 300 yards we would flip the 'long range' sights up.

The A2 does not have a long range sight, it has a limited visability site that is manipulated the same way that the long range sight on the A1 was, I think this has lead to a lot of confusion.

This 5mm (0-2) rear aperture was also intended to be used wth the then current Army Low Light Llevel System that included a luminous front sight post and a two-leaf aperture for the M16A1. But this aperture was never very popular because an Armorer had to install it, and then you lost the long range aperture from the standard A1 two-leaf function.
So by incorporating this 5mm ap in the A2, it was ready for the Operator to switch to the LLL luminous front sight post which he could do himself without any special tools.

[Molon]

I think the person that revised the field manual probably was just doing a 'check the box' to get the publication to press.

They should have been more specific, the A1 does have a long range sight, the apeture is the same size. When we would shoot further than 300 yards we would flip the 'long range' sights up.

The A2 does not have a long range sight, it has a limited visability site that is manipulated the same way that the long range sight on the A1 was, I think this has lead to a lot of confusion.

The manual was very specific. The sectioned I quoted from was specifically referring to the A2 sight.

[mos2111]

Found this, it is extremely relevant to your topic.
http://www.usaac.army.mil/amu/Servic...-M4%20data.pdf

[bobafett]

So in practice, when you zero for less than 300 meters do you never use the elevation wheel on the A2 sight?

Now I know why I just keep a BUIS on my rifle and it's zeroed for 50yds.

My head hurts

[bobafett]

I guess my confusion in it all is that I want to think a 14.5" and a 20" barreled gun should require different elevation wheels to begin with.

Edited..... I read the revised improved battlesight zero thread on AR15.com I see you don't lose your adjustments at all, sorry .