Carbine Basics

[rob_s]

These methods are still entitled "Field-Expedient Zero" in the pubs, though there are instances in which the word "zero" is left out entirely on the offhand chance of discouraging those afflicted with Brig-Lawyer Syndrome ("...it SAYS 'zero' right THERE..."), and the urge to half-step their way through life, from replacing an actual zeroing process with the short-range methods. This is on the Corps side of the house, don't know about Army or other organizations, but some publications where the field-expedient methods will now read "XYZ m/yd Grouping Exercise" are gonna become more common. I think it's "happy vs. glad" nonsense, but nobody asked me at the time, either, so... .

I think that's fantastic. It's an initial intersection, or at best a "near zero", not a "50 yard zero".

[Failure2Stop]

I think that's fantastic. It's an initial intersection, or at best a "near zero", not a "50 yard zero".

Rob, I'm a bit interested where this comes from. For as long as I have been a rifle shooter any distance at which one has a definitive point of impact in known relation to point of aim, with no environmental effect, has been referred to as a "zero".
So one could have a point of aim/point of impact intersection at 7 yards, which would be a "7 yard zero", or conversely, point of impact being 3" above point of aim at 100 meters, which would be a " 3" high at 100 meter zero".
The argument in the Corps was about what a "BZO" (battle sight zero) entailed. Really, it's a 300 meter zero, which theoretically permits a shooter to hit human sized targets from muzzle to 350 meters without holding off. This morphed into a 300 yard zero, since the vast majority of KD ranges are in yards, and most US shooters think in yards, not meters (though this is slowly but steadily changing). To prepare shooters for zeroing at 300, an initial shoot at 36 yards (for 20" barrels shooting M855) would be done to "get on paper" at 300. Somehow, the 36 yard shoot got instilled as "BZO ing" at an institutional level. In reality, it is a method to prepare the shooter for the "real zero/BZO", or to get "close enough" should something happen to cause loss of zero. In this instance, the "field expedient BZO" is not a BZO, but rather a 36 yard zero.

Typos brought to you via Tapatalk and autocorrect.

[rob_s]

Rob, I'm a bit interested where this comes from. For as long as I have been a rifle shooter any distance at which one has a definitive point of impact in known relation to point of aim, with no environmental effect, has been referred to as a "zero".
So one could have a point of aim/point of impact intersection at 7 yards, which would be a "7 yard zero", or conversely, point of impact being 3" above point of aim at 100 meters, which would be a " 3" high at 100 meter zero".
The argument in the Corps was about what a "BZO" (battle sight zero) entailed. Really, it's a 300 meter zero, which theoretically permits a shooter to hit human sized targets from muzzle to 350 meters without holding off. This morphed into a 300 yard zero, since the vast majority of KD ranges are in yards, and most US shooters think in yards, not meters (though this is slowly but steadily changing). To prepare shooters for zeroing at 300, an initial shoot at 36 yards (for 20" barrels shooting M855) would be done to "get on paper" at 300. Somehow, the 36 yard shoot got instilled as "BZO ing" at an institutional level. In reality, it is a method to prepare the shooter for the "real zero/BZO", or to get "close enough" should something happen to cause loss of zero. In this instance, the "field expedient BZO" is not a BZO, but rather a 36 yard zero.

Typos brought to you via Tapatalk and autocorrect.

After the multiple shitstorms that ensued on M4C discussing this, I don't really see the point. When those discussions happened I was able to use google to find explanations of what I was trying to get across, but which people were too busy debating the person and not the topic and too busy getting worked up that something they previously held to be true may not be so, to make it worthwhile even to post those links at the time, or bother looking for them now.

I will, however, say that I think that your description in the second paragraph is largely what led people to the colloquialism that passes for technical terminology today. Combined with many people who's only (or at least, initial and long-term) familiarity with shooting has come from military service.

In either case, I think it's hard to debate the usefulness of having a different term for near- and far-zeros, even if we have to call them "purple dinosaur" and "yellow bird" (respectively) just to make people feel better, and would help to avoid the confusion altogether.

[MD7305]

Holy Moses!!!, this is an old thread but seemed the most appropriate place for my question. I have some carbine (AR/AK) experience but I know very little and have only very basic training with carbines. I recently built a new carbine; DD mid upper a/Troy Alpha, DD BUIS, EOTech 512 on top of a Spikes complete lower. My initial zero was done at 25 yards due to range limitations. Everything was great, irons and EOTech we're making cloverleaf groups. So today I went to a 200yd range to adjust/verify my zero after adding a YHM 1/3rd riser to the 512. I started at 50yds and my rounds were hitting high, right (think top right corner of a 8.5x11 sheet of paper). I worked both irons and EOTech back to center. I stopped at 50yds because I had a bunch of questions and didn't want to waste my ammo. So here are my questions regarding factors that effect zero. Would these factors have such a great effect on the difference in zero? I fully expected it to widen due to distance and the riser but not such a pronounced change. Nothing was touched between both zeroes except adding the riser.

-My original zero was done prone and second was from a bench.
-Not sure if stock was collapsed at the same length each time?
-Riser seemed solid and tight, is it junk or a weak link?
-Ammo: XM193 first time, Federal bulk 55gr. today. Velocity related?
-Doofus behind the trigger. Put the carbine down and stick to pistols?

Any suggestions welcome.

[Jay Cunningham]

Understand that the "50/200" zero is not exactly that.

You need to pick something and keep it consistent first.

My recommendation is to pick the bench, pick the fully extended stock, pick one ammo type, and pick your EoTech with the illumination adjusted down so that you can barely see it.

Get back on a target at 25 yards. You can zero however you like eventually, but you need to tighten it up at 25. I use a rough zero of hitting 1.5" low at 25 to get me rough zeroed at 100.

Work on keeping a constant body position behind the gun on the bench. Employ breath control and center up the dot in the optic (yes they're supposed to be parallax free, but we're tweaking stuff here).

To be honest, all of the things you mentioned should only make a small bit of difference.

[Jay Cunningham]

Also... are you sure you're using the right riser?

I went back an re-read your post... you changed a lot of variables. You need to re-zero.

[rob_s]

Holy Moses!!!, this is an old thread but seemed the most appropriate place for my question. I have some carbine (AR/AK) experience but I know very little and have only very basic training with carbines. I recently built a new carbine; DD mid upper a/Troy Alpha, DD BUIS, EOTech 512 on top of a Spikes complete lower. My initial zero was done at 25 yards due to range limitations. Everything was great, irons and EOTech we're making cloverleaf groups. So today I went to a 200yd range to adjust/verify my zero after adding a YHM 1/3rd riser to the 512. I started at 50yds and my rounds were hitting high, right (think top right corner of a 8.5x11 sheet of paper). I worked both irons and EOTech back to center. I stopped at 50yds because I had a bunch of questions and didn't want to waste my ammo. So here are my questions regarding factors that effect zero. Would these factors have such a great effect on the difference in zero? I fully expected it to widen due to distance and the riser but not such a pronounced change. Nothing was touched between both zeroes except adding the riser.

-My original zero was done prone and second was from a bench.
-Not sure if stock was collapsed at the same length each time?
-Riser seemed solid and tight, is it junk or a weak link?
-Ammo: XM193 first time, Federal bulk 55gr. today. Velocity related?
-Doofus behind the trigger. Put the carbine down and stick to pistols?

Any suggestions welcome.

if I'm reading this right, you got a near-zero at 25 yards with an Eotech mounted directly to the flattop, then installed a YHM riser, went and shot at 5o yards and found that your groups were both high and right of your point of aim?

If so, there are two components here...
(1) the bullet is traveling upward, relative to your line of sight, as it impacts the target at 25 yards. It will still be traveling upwards at 50 yards, meaning it will hit high.
(2) changing the mounting can affect the lateral point of aim (the bullet still travels in the same path) even if the literature all says it won't.

Other factors include basic geometry (a .25" shift to the right at 25 yards may be invisible but will start to show up as a .50" shift at 50, 1" shift at 100, and 2" shift at 200), and the fact that this is why we shoot to the distances, not rely on the theory. Will a 2" deviation at 200 mean you miss the steel? No. But if you're trying to shoot a group at 200 on paper you can see the shift.

[joshs]

-My original zero was done prone and second was from a bench.

Changing positions can affect zero, especially with irons or an optic at a distance other than the parallax is set for.

-Not sure if stock was collapsed at the same length each time?

Same as above, anything that changes your eye's relationship to the sights/optic can affect zero.

-Riser seemed solid and tight, is it junk or a weak link?

Maybe, but the best way to find out is to get the a good zero and see if there is any shift over time. Ensure that the connections are properly to torqued and mark with a paint pen so you'll see if there is any movement in the screw/s.

-Ammo: XM193 first time, Federal bulk 55gr. today. Velocity related?

Changing ammo, even lot to lot of the same ammo, can affect zero.

-Doofus behind the trigger. Put the carbine down and stick to pistols?

I think it's very possible that all of the above could have led to the shift that you saw. You'll normally be a little high at 50 with a 25 yard zero; add in all of the changes you made, and the shift could easily be explained away. I'd try to get a new zero, shoot for a bit, and then recheck zero from the same position to eliminate any variables.

[Dave J]

Nothing was touched between both zeroes except adding the riser.

To me, that alone represents a very significant change, and I'm not surprised you saw a very noticeable shift.

-Ammo: XM193 first time, Federal bulk 55gr. today. Velocity related?

Not surprising to couple MOA worth of change there as well. Individual rifles vary with how much they'll shift between different loads.

FWIW, I've also seen EoTechs shift zero seemingly of their own accord...not a lot, but a few clicks difference from week to week wouldn't surprise me, especially if the rifle's been beaten around a bit.

[JMS]

Jay effectively said what I was typing out, the action of dismounting your optic and adding the riser effectively taking you back to Square One, regardless of by how much your POI shifted. You were gonna see some differences, and the fact that there's something like 6 variables present in what you just got through posting...

Put your initial effort out of your head, it's irrelevant, at this point.

If you're not eliminating as many variables as you can, you're injecting variables into an exercise where variables are unwelcome. Follow Jay's words in this regard; zeroing is largely administrative in nature, and it's worth treating it as such...though there's no need to go down the rabbit-hole over it, like the bench-rest competitors do. (Which is not a bust on the bench-resters; the high level of precision and attention to detail they go through in zeroing feeds directly in to their intended usage.)

Going from 25yd to 50, you're POI would have been a bit high even if you'd not made a drastic equipment change....but you did. The equipment change AND the 3-4 other conditions that may or may not have altered from your initial effort all came together as one, in this instance. Simplest to just start over.

A greater understanding of the external ballistics of the various most-popular zeros would be a worthwhile endeavor. Just about anything Molon's ever posted is very worthwhile reading: http://www.m4carbine.net/showthread....d-Trajectories