100m Zero

[Armati]

Please explain the 100m zero. In the military we are fond of doing a simulated 25m zero for 300m. However, a lot of very knowledgeable instructors want a 100m zero for their class. Please explain this. Thanks.

[Redhat]

http://m4carbine.net/showthread.php?t=65679

Here you go...enjoy!

[RogerinTPA]

IRRC, the popularity came from instructors that were former Army SMU types (LAV, Tigerswan, Lamb, etc...) who stated that 100M or yard zero, was the most effective zero for common engagements for that unit's mission (counter terrorism). So they advocate that zero in their classes. There are other well known instructors outside that particular group, who advocate the 50M/yard/200 zero for its flatter trajectory out to 220 yards or so, thus being more efficient. Pretty much what a lot of folks here use, including myself. The Army's 300 yard zero, as you know, was based on a static defense from the Cold war era, while reducing the qualification time and resources for regular soldiers at 25 yards. Big Army still hasn't made the leap to a more common threat engagement range zero, the 50/200 zero, which many Infantry types are deeming more practical. An Infantry Major wrote a paper while at Leavenworth entitled "Taking back the Infantry half Kilometer" which stirred up a hornet's nest, basically slapped down the current training and zeroing process.

[a0cake]

The 100M zero offers minimal offsets before maximum ordinate, which conveniently coincides with the primary (and only) zero. Consequently, there are no hold-unders, except for extreme angle shots. All of this translates into an ability to be extremely precise in placing a shot exactly where you want it from 0 - 100M.

At 200M, you're looking at only around 3.5'' of drop with M855A1 out of a 14.5'' barrel.

At 300M, there is just over 12 inches, an easy reference.

Past that, it does get a little dicey with a RDS or other non-BDC reticle.

At 400M, drop is about 3 feet.

At 500, there's an entire upright man's worth of drop, at 70''.

So, with iron sights or a RDS you're clearly into some relatively extreme holdovers at extended range. This is a trade off of the 100M zero.

Now, let's look at a 25M zero at these ranges.

At 300, you're actually still going to be a bit above LOS, usually about 3 inches high still. At 400, you're a foot low. At 500, you're 3 and a half feet low.

As you can see, this is slightly more forgiving at intermediate distances.

However, the trade off is offsets at close range. At 170-200M, you're a full 8 inches high with the 25M zero.

Where you run into trouble is steep vertical angles like we're seeing in Afghanistan. At 200M, for example, with a 25M zero you're already shooting 8 inches high. Add a 65 degree vertical angular deviation to that you and you've doubled your holdunder to almost 16 inches.

At the same range (200M), as we said earlier the drop from a 100M zero requires a 3.5 inch holdover. Add a 65 degree vertical angle and now you need a 3.5 inch holdunder. At worst, you're going to be 3.5 inches off if you aim dead on. That's almost 5X less error compared with the 25M zero.

Following?

I personally use and advocate a 100M zero for all optics. Why? Because we get ACOG'S and Elcan Spectre DR's issued to us. Both use BDC's based off a 100M zero. You spend enough time on the range to know what appropriate holdovers from a 100M zero look like. It's too easy to transition over to the Aimpoints and EOTECHS and use the same holdovers.

I also like the minimal close range offsets. There's nothing like being able to pick which eye you want to shoot and then putting a round through it. Also good for shooting through peepholes / loopholes that shitheads often blind fire through. Putting a round through a small slit in a dirt wall at 110M with a 25M zero can be a little bit hard. With a 100M zero it's a no brainer...literally.

That's my reasoning at least. The point is to pick a zero, then learn it inside and out. I'm reciting all these offsets from memory. It's important to know your trajectory. Get to the point where you can do the same, and you'll be effective with whatever zero you choose.

[Armati]

http://m4carbine.net/showthread.php?t=65679

Here you go...enjoy!

Thanks a lot.

So... it would seem that for most urban shooting the 100m zero might be the preferred solution.

[Winston Wolf]

Out-friccin-Standing explanation! Masterful even.

Well said Sir.



The 100M zero offers minimal offsets before maximum ordinate, which conveniently coincides with the primary (and only) zero. Consequently, there are no hold-unders, except for extreme angle shots. All of this translates into an ability to be extremely precise in placing a shot exactly where you want it from 0 - 100M.

At 200M, you're looking at only around 3.5'' of drop with M855A1 out of a 14.5'' barrel.

At 300M, there is just over 12 inches, an easy reference.

Past that, it does get a little dicey with a RDS or other non-BDC reticle.

At 400M, drop is about 3 feet.

At 500, there's an entire upright man's worth of drop, at 70''.

So, with iron sights or a RDS you're clearly into some relatively extreme holdovers at extended range. This is a trade off of the 100M zero.

Now, let's look at a 25M zero at these ranges.

At 300, you're actually still going to be a bit above LOS, usually about 3 inches high still. At 400, you're a foot low. At 500, you're 3 and a half feet low.

As you can see, this is slightly more forgiving at intermediate distances.

However, the trade off is offsets at close range. At 170-200M, you're a full 8 inches high with the 25M zero.

Where you run into trouble is steep vertical angles like we're seeing in Afghanistan. At 200M with a 25M zero you're already shooting 8 inches high. Add a 65 degree vertical angular deviation to that you and you've doubled your holdunder to almost 16 inches.

At the same range (200M), as we said earlier the drop from a 100M zero requires a 3.5 inch holdover. Add a 65 degree vertical angle and now you need a 3.5 inch holdunder. At worst, you're going to be 7 inches off if you don't account for angle. That's less than half the error with the 25M zero.

Following?

I personally use and advocate a 100M zero for all optics. Why? Because we get ACOG'S and Elcan Spectre DR's issued to us. Both use BDC's based off a 100M zero. You spend enough time on the range to know what appropriate holdovers from a 100M zero look like. It's too easy to transition over to the Aimpoints and EOTECHS and use the same holdovers.

I also like the minimal close range offsets. There's nothing like being able to pick which eye you want to shoot and then putting a round through it. Also good for shooting through peepholes / loopholes that shitheads often blind fire through. Putting a round through a small slit in a dirt wall at 110M with a 25M zero can be a little bit hard. With a 100M zero it's a no brainer...literally.

That's my reasoning at least. The point is to pick a zero, then learn it inside and out. I'm reciting all these offsets from memory. It's important to know your trajectory. Get to the point where you can do the same, and you'll be effective with whatever zero you choose.

[Savior 6]

Thanks a0cake. That was very helpful and to the point.

[jenrick]

I'm a big fan of starting rifle shooters off with the 100m zero when I introduce them to distance shooting. They only have to hold over, which is natural in instinctive act, we don't worry about holding under when we throw a ball. We simply aim high and throw, same with the rifle with a 100m zero. It's easy to teach hold overs that way before you start to look at other zero's that require hold unders and varying distances.

-Jenrick

[glocktogo]

The 100M zero offers minimal offsets before maximum ordinate, which conveniently coincides with the primary (and only) zero. Consequently, there are no hold-unders, except for extreme angle shots. All of this translates into an ability to be extremely precise in placing a shot exactly where you want it from 0 - 100M.

At 200M, you're looking at only around 3.5'' of drop with M855A1 out of a 14.5'' barrel.

At 300M, there is just over 12 inches, an easy reference.

Past that, it does get a little dicey with a RDS or other non-BDC reticle.

At 400M, drop is about 3 feet.

At 500, there's an entire upright man's worth of drop, at 70''.

So, with iron sights or a RDS you're clearly into some relatively extreme holdovers at extended range. This is a trade off of the 100M zero.

Now, let's look at a 25M zero at these ranges.

At 300, you're actually still going to be a bit above LOS, usually about 3 inches high still. At 400, you're a foot low. At 500, you're 3 and a half feet low.

As you can see, this is slightly more forgiving at intermediate distances.

However, the trade off is offsets at close range. At 170-200M, you're a full 8 inches high with the 25M zero.

Where you run into trouble is steep vertical angles like we're seeing in Afghanistan. At 200M, for example, with a 25M zero you're already shooting 8 inches high. Add a 65 degree vertical angular deviation to that you and you've doubled your holdunder to almost 16 inches.

At the same range (200M), as we said earlier the drop from a 100M zero requires a 3.5 inch holdover. Add a 65 degree vertical angle and now you need a 3.5 inch holdunder. At worst, you're going to be 3.5 inches off if you aim dead on. That's almost 5X less error compared with the 25M zero.

Following?

I personally use and advocate a 100M zero for all optics. Why? Because we get ACOG'S and Elcan Spectre DR's issued to us. Both use BDC's based off a 100M zero. You spend enough time on the range to know what appropriate holdovers from a 100M zero look like. It's too easy to transition over to the Aimpoints and EOTECHS and use the same holdovers.

I also like the minimal close range offsets. There's nothing like being able to pick which eye you want to shoot and then putting a round through it. Also good for shooting through peepholes / loopholes that shitheads often blind fire through. Putting a round through a small slit in a dirt wall at 110M with a 25M zero can be a little bit hard. With a 100M zero it's a no brainer...literally.

That's my reasoning at least. The point is to pick a zero, then learn it inside and out. I'm reciting all these offsets from memory. It's important to know your trajectory. Get to the point where you can do the same, and you'll be effective with whatever zero you choose.

Best explanation I've seen yet. Kudos!

[Armati]

Now, riddle me this:

How do you recommend one achieves a 100m zero using your typical Army range facilities? Most zero ranges are only 25m. Would you use a KD range and spotting scope/range walk?

[a0cake]

Now, riddle me this:

How do you recommend one achieves a 100m zero using your typical Army range facilities? Most zero ranges are only 25m. Would you use a KD range and spotting scope/range walk?

Where I was at, there was almost never an issue finding a 100M piece of real estate up-range from the zero line. I'm not sure how spacious the range facilities are Army wide. In some instances you just have to tell range control to suck it. But most of the time they'll work with you when it comes to backing off the 25M line to get zeroed.

The harder part for most people, I think, will be getting the upper echelon leadership to even think about alternatives to the 25M zero. If that can be accomplished, the practical issues of actually doing it can be easily overcome.

[Redhat]

Just out of curiosity...has the AMU provided their opinion on this? Would it help if they supported it?

[nickdrak]

The general rule to get a rough 100yd zero when limited to a 25yd range is to adjust your point of impact to 1.5" directly below your point of aim @ 25yds.

You must then go out and confirm @ 100yds.

[Redhat]

Funny...I'll bet most don't bother to "confirm" the 25m/300m either.

[Tzoid]

Great post..

For the last 4 years I have been running a 50 yard Zero but after a class with Kyle Defoor back in June I quickly moved to a 100 yard Zero.

I say the Mods need to make this a Skicky :D

[nickdrak]

Funny...I'll bet most don't bother to "confirm" the 25m/300m either.

I firmly believe the Army does it @ 25m because it is easy to get thousands of 18 & 19 year old kids on paper @ 25m. Not because of any practical application to combat.

[glocktogo]

I firmly believe the Army does it @ 25m because it is easy to get thousands of 18 & 19 year old kids on paper @ 25m. Not because of any practical application to combat.

Winner, winner, chicken dinner...