M16A2 sight in process and general questions

[lysanderxiii]

Here is a graphic of the trajectory of M855 using various zero methods.

- The blue line with triangles is the trajectory when you put a target out at 300 meters and shoot at it until your point of aim and point of impact coincide.
- The yellow line with crosses is the trajectory if you set your M16A2 rear sight at 8/3 plus two clicks, put a target out at 25 meters and shoot at it until your point of aim and point of impact coincide.
- The green line with squares is the trajectory if you set your M16A2 rear sight at 8/3, put a target out at 36 yard (33 meters) and shoot at it until your point of aim and point of impact coincide.
- The red line with circles is the trajectory if you set your M16A2 rear sight at 8/3, put a target out at 25 meters and shoot at it until your point of aim and point of impact coincide.

As you can see the green line and the yellow line are pretty much and exact match.

In the M4 graphic the blue line is the trajectory when you put a target out at 300 meters and shoot at it until your point of aim and point of impact coincide.
The orange is if you follow the Army's zero procedure, as described in FM 3-22.9. The green line is the trajectory if you follow the 36 yard zero procedure as described in the USMC manual MCRP 3-01A.

It should be also noted that MCRP 3-01A has this to say about reduced rage zeroing:

"Zeroing is conducted at a range of 300 meters/yards. If a 300-meter/yard range is not available, a prezero sight setting can be established at a reduced range of 33 meters/36 yards. This does not constitute a BZO. The process at this short range allows the Marine to “get a group” on paper in preparation for firing at 300 meters/yards. When a Service rifle is zeroed for 300 meters/yards, the bullet crosses the line of sight twice. First, it crosses the line of sight at 33 meters/36 yards and then again farther down range at 300 meters/yards as it reenters the line of sight (see figs. 8-20 and 8-21)."

So, the fact that the 36 yard reduce range zero yields results that are 2 inches low is acceptable as this will be refined when shooting is done at 300 meters.




All calculations were done with JBM Ballistics' on-line trajectory calculator, using the following parameters:

M855 Ball:
Weight: 62 grains
BC: 0.151 (G7, from ARL data)
Velocity-
- M16A2: 3020 fps at 78 feet (from MIL-DTL-63989)
- M4: 2910 fps at muzzle (from Army Evaluation Center tests of M4)
Sight height-
- M16A2: 2.59 inch
- M4: 2.63 inch

Temperature: 70 degrees F
Pressure: 29.92 inHg
Humidity: 50%

And, one click elevation of an M16A2 rear sight know is equal to 3/8 inch at 25 Meter, per the Army's 25 meter zero target.

Raw data:

 

[12Bravo20]

And as your chart how the optimal battle sight zero is 36 yards out to 300 yards or 300 meters. I have had similar results actually shooting at those different ranges using both M193 and M855 ammo out of a 20" A2 barrel. There was a difference between the two bullet weights of course.

Past 300 yards/meters is when the rear sight elevation comes into play. And yes it definitely works as designed. I have done it quite a bit.

 

[lysanderxiii]

Actually the person that made that chart and associated video was using a red dot sight.

If he used a red dot then he cannot have put in the proper 2 click offset and that it not a "300 meter" zero. It is something else that we do not know what is, as the sight height (height of the centerline of the scope above the bore line) is a crucial piece of information when doing these reduced range zeros.

People seem to think that all 25 meter zeros are the same; they are not. Even with the same ammunition, sight height and barrel length make huge differences in where the bullet crosses your line of sight on the downward leg of its journey. For example, on the same rifle, the difference in zero between a red dot with the reticle co-witnessed with the front sight or a one-third witness, is nearly 50 yards and a maximum ordinate difference of almost 3 inches.

 

[lysanderxiii]

And as your chart how the optimal battle sight zero is 36 yards out to 300 yards or 300 meters. I have had similar results actually shooting at those different ranges using both M193 and M855 ammo out of a 20" A2 barrel. There was a difference between the two bullet weights of course.

Past 300 yards/meters is when the rear sight elevation comes into play. And yes it definitely works as designed. I have done it quite a bit.

My chart also shows that a 25 meter plus two clicks yields the exact same trajectory. Note the green (36 yard) and yellow (25 m +2) are right on top of each other.

Also, this is all for the small aperture, once you flip it to the big aperture you are at a 200 meter zero.

 

[robin banks]

what I have found and is just common sense is that sighting in at 25 yds and expecting to be on at 300 yds is not gonna happen. at 25 yds no imperfections as far as left and right will show up. site in at 2 or 300 yds loosen and move the drum to 3 if sighted in a 200 yds move the drum a few clics before 3. adjusting the front site sometimes make it easier

 

[JCSC]

I gave the 20” A2 a run yesterday afternoon. It was extremely hot and I didn’t really sight it in per the process. It was pretty close at 100, so I made a slight windage adjustment and shot a full mag.

This was Hornady 2267 FMJ over H335. My blasting load. I shot all of the rounds at a moderate pace and I’m not really disappointed with the 100 yard results.

This ammo doesn’t usually shoot better than 1.5” with a scope and sled.

I had a handful on the right side as I made my adjustment, but that a 3-4” grouping. I could do much better I am sure.

rifle length gas and long barrel makes this a really smooth shooter

 

[DMW1116]

Do you have a standard front post or a smaller/narrower one?

 

[JCSC]

Not certain on that. I believe it is a square profile.

it certainly cover up a lot of the target at 100 yards.

 

[DMW1116]

I think the standard widths are 0.072”. My KNS Variety Pack has that, a 0.052”, and a 0.032” post. The 0.072” matches a piece of notebook paper horizontally at 100 yards so windage is easy to set. Elevation is still tricky. That’s how I tested hand loads until I switched to the KNS pointed pyramid topped post. I like that better but I think I’m in the minority on that.

 

[robin banks]

Not certain on that. I believe it is a square profile.

it certainly cover up a lot of the target at 100 yards.

once elevation is set file each side until it is what you want

 

[Riomouse911]

For battle-type zeriong this chart is fairly accurate.

I had my agency zero at 50 yds. With our particular guns-ammo that should give them the ability to hit a suspects torso out to 200-225 yards with a center of mass hold. We have a LOT of open desert here with illegal marijuana grows everywhere, so a 200-300 yard gunfight is not out of the realm of possibilities.

Of course the vast majority of duty-carbine shots are within 100 yards. Our experience showed the 50 yard zero needs no holdover adjustment until you are really reaching out.

This works for us, YMMV.

Stay safe.

 

[lysanderxiii]

View attachment 1094739

For battle-type zeriong this chart is fairly accurate.

I had my agency zero at 50 yds. With our particular guns-ammo that should give them the ability to hit a suspects torso out to 200-225 yards with a center of mass hold. We have a LOT of open desert here with illegal marijuana grows everywhere, so a 200-300 yard gunfight is not out of the realm of possibilities.

Of course the vast majority of duty-carbine shots are within 100 yards. Our experience showed the 50 yard zero needs no holdover adjustment until you are really reaching out.

This works for us, YMMV.

Stay safe.

A 50 yard zero with M193 or M855 is pretty much a 200 yard zero.

If you follow all the procedures in FM 3-22.9, when you're all done you will essentially have a 200 yard zero with the big aperture. And when qualifying, only the 300 target required anything other than a CM hold.

 

[JCSC]

Do the large and small aperture have different vertical centerlines?

 

[lysanderxiii]

Do the large and small aperture have different vertical centerlines?

On the "L-type" M16A2 rear sight, the center of the small aperture is centered on the line on the top of the big aperture.

The center of the large aperture is offset to the right by 0.007 +/-0.002" to take into account the rotation of the sight on the threaded windage screw will move the sight to the left.

The thread is 36 threads per inch, or 0.0278" per revolution. Flipping from the large aperture to the small aperture rotates the sight 90 degrees, so . . .

0.0278 / 4 = .00694”

The old M16A1 rear sight has a similar 0.007" offset between the "L" and plain aperture.