Carbine Basics

[Suvorov]

For what it is worth, I have stopped zeroing my rifles from the bench. I have found that I get quite a bit of difference between my POI on a 100 yard zero shot from a bench and a 100 yard zero I shoot from prone (about 3-4 inches) simply because I mount the rifle differently when I'm laying down and when I'm sitting. Whatever theoretical improvement in stability and precision I gain from the bench is lost when I change to shooting from positions and since I seldom shoot from the bench and would not have a bench in the field (whether it be matches, goblins, or alien invasion), it does me no good to zero from the bench. The bench to prone zero shift resulted in a mostly vertical change in POI. I would also notice a windage POI shift from time to time that was due to me canting the rifle differently from the bench to prone. To eliminate this I now shoot with my front sight post up as by making sure the dot or reticle is centered above my front sight post - I eliminate any cant I may be inducing.

As the others have mentioned, your installing the riser and changing ammo were huge variable changes that would result in you needing to re-zero, but it is wise to also take shooting position into account.

[HCM]

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.

This has been my experience with our EO-Tech 552s at work, especially if they are knocking around in a lock box in a vehicle. We run them directly on the rail with the small screw, Lock Tite and witness marks. I would also recommend lithium AAs for your 512.

[MD7305]

Thanks for the advice gents. I had a Primary Arms red dot on the carbine for little while as a temporary optic until I found something more appropriate. I had no issues with it but I initially zeroed at 25yds, prone. I think today I had too many variables going on. I'm going to start fresh next time out and make a note to mark my stock position, stick to prone, and stick with XM193. I know zero is a debated issue, at least distance. I usually do a 25yd zero with POA about 1.5" above POI as Jay mentioned earlier. This is mainly due to my departments range being limited to 25yds. This time out I was going for a 50yd zero, with really no justification other than curiosity. You guys made several points I hadn't considered so thanks again.

[Jay Cunningham]

Once you're settled in, please don't overlook tightening down and putting thread locker on stuff.

[Molon]

A good zero is vital to rifle shooting -- whats the best option for a general use carbine (16 inch, irons/1x red dot), and how do we get there?

AR-15 Zeros and Trajectories


The 100 yard zero with a .223/5.56mm AR-15 carbine is a unique trajectory in that the bullet just “kisses” the line of sight at 100 yards and rides along it for approximately 10 yards before dropping back down below the line of sight. (Technically, the bullet does travel above the line of sight, but by only 0.010”; a fraction of the diameter of the bullet itself.)







For all other zeroing schemes, there are going to be two points were the bullet crosses the line of sight; the near-zero and the far-zero. For the near-zero, the bullet will cross the line of sight while traveling upward form the muzzle toward the apogee or “maximum ordinate,” its highest point of travel. For the far-zero, the bullet will cross the line of sight while traveling downward from the maximum ordinate.

Now, when assigning a name to a particular zeroing scheme, it would be helpful if that name gave descriptive information about that particular zero; that is, the name should give us information about the trajectory and how it is unique and differs from other trajectories.

As a point of reference, the Santose Improved Battlesight Zero is often referred to as a 50/200 yard zero, however this is incorrect. It is actually a 50 yard/200 meter zeroing scheme; and this is only with a very few particular combinations of bullet weight, barrel length/muzzle velocity and height of sights above the bore. As an example, a 20” barreled AR-15 A2 firing 62 grain M855 will not match the 50 yard/200 meter IBZ. Neither will a 16" barreled RECCE firing 77 grain MK262, nor a 14.5” barreled M4 carbine firing the 70 grain 5.56mm Optimized "Brown Tip" load. The same concept applies when people refer to a 50/225 yard zero. Only a very few specific combinations of bullet weight, barrel length/muzzle velocity and height of sights above the bore will match that description.

What this is all leading up to is this; except for a very few specific combinations of bullet weight, barrel length/muzzle velocity and height of sights above the bore, a 50 yard zero is a different zero than a 200 yard zero. For a 200 yard zero, we know that this trajectory will produce a far-zero in which the bullet will cross the line of sight at 200 yards in its downward travel from the maximum ordinate. (It is physically impossible to produce a 200 yard near-zero with any of the commonly available loads and barrels lengths used in .223/5.56mm AR-15s.) Other than for a very few specific combinations of bullet weight, barrel length/muzzle velocity and height of sights above the bore, the near-zero of the 200 yard zero will not be at 50 yards.

Conversely, a 50 yard zero tells us that this trajectory will have a near-zero in which the bullet crosses the line of sight at a distance of 50 yards in its upward travel to the maximum ordinate. For those who think that a zeroing scheme must be named after its far-zero, it is physically impossible to produce a far-zero of 50 yards with any of the commonly used loads and barrel lengths in .223/5.56mm AR-15s. The 50 yard zero can only be the near-zero.

Other than for a very few specific combinations of bullet weight, barrel length/muzzle velocity and height of sights above the bore, the far-zero of the 50 yard zero will not be at 200 yards; and for all practical purposes it matters not one bit. Whether the bullet crosses the line of sight for the second time (far-zero) at 189 yards, 200 yards, 215 yards or 225 yards will not make the slightest bit of difference in the practical application of the AR-15 as a defensive weapon. In each case we will be holding the same POA (beyond CQB distances) and know that we will be hitting within approximately 2 inches above or below that POA out to 200 yards (or farther depending upon barrel length and load.) You should have an idea what your actual far-zero is when using a 50 yard zero and confirm such at distance when possible, but again it’s most likely not going to be a 200 yard far-zero and again it does not need to be.

Choose your zeroing scheme based on the pertinent facts; not nonsense about “shooting through a cone.” When shooting at human targets, in the grand scheme of things there isn’t going to be any practical difference between a point of impact that has a negative deviation from the point of aim, (e.g. the bullet strikes 1.5” below the point of aim) and a point of impact that has an equal positive deviation from the point of aim (e.g. the bullet strikes 1.5” above the point of aim.) In other words, the absolute value of the point of impact from the point of aim (how far the point of impact deviates from the point of aim, regardless of whether it is a positive or negative deviation) is what we need to be concerned about. Therefore, one of the main points to consider when choosing a battle-sight-zero is this: What zeroing scheme produces the smallest absolute values for the deviations of the points of impact from the point of aim, over the distance that we reasonably expect to engage a human target in our intended usage?

The chart below illustrates the above concept. The chart compares the absolute values of the deviations of the points of impact from the point of aim (0.0 inches on the graph being the point of aim/line of sight) for a 50-yard-zero and a 100-yard-zero, using Hornady 5.56 TAP T2 ammunition.





As you can see in the graph above, from the muzzle (0 yards) to approximately 62 yards, the 50-yard-zero has a slight advantage over the 100-yard-zero. Between the distances of 62 yards and 165 yards, the 100-yard-zero has the advantage. From the distance of 165 yards out to the 250 yards shown in the graph, the 50-yard-zero has a distinct advantage over the 100-yard-zero. Choose your zeroing scheme based on the pertinent facts.




Some reference material. Except where noted, all barrel lengths are 20 inches



Courtesy of zrxc77





M855 25 yard zero







M855 25 meter zero






M855







M855 and M193 50 yard zero from 16” barrel







M193 25 meter, 50 yard and 100 yard zeros from 16" barrel






100 yard zero








M855 200 meter vs 300 meter zeros





....

[JMS]

^^Molon's immediate above is precisely the info I cited, RE: info on types of zero.

General observation: A key portion of it is often glossed over by a lot of folks who falsely claim to understand basic zeros, largely because they fail to shake the misguided idea of "yards and meters are the same thing," or analogues thereof...

Bolded portion is my emphasis:

As a point of reference, the Santose Improved Battlesight Zero is often referred to as a 50/200 yard zero, however this is incorrect. It is actually a 50 yard/200 meter zeroing scheme; and this is only with a very few particular combinations of bullet weight, barrel length/muzzle velocity and height of sights above the bore. As an example, a 20” barreled AR-15 A2 firing 62 grain M855 will not match the 50 yard/200 meter IBZ. Neither will a 16" barreled RECCE firing 77 grain MK262, nor a 14.5” barreled M4 carbine firing the 70 grain 5.56mm Optimized "Brown Tip" load. The same concept applies when people refer to a 50/225 yard zero. Only a very few specific combinations of bullet weight, barrel length/muzzle velocity and height of sights above the bore will match that description.

What this is all leading up to is this; except for a very few specific combinations of bullet weight, barrel length/muzzle velocity and height of sights above the bore, a 50 yard zero is a different zero than a 200 yard zero.

100 yards is 100 yards. 100 meters, however, is 109 yards and change; over 27' of difference. Most of the difference between a 15yd pistol target and a 25yd pistol target, which gives all kinds of folks all manner of troubles....

I don't have enough ink in my pen to describe all the troubles we had with USMC RCOs that were zeroed @ 100yd, and the Marines insisting that the optic was a POS because the BDC aimpoints at further distances (an ACOG BDC is graduated in meters) didn't match their zeros. Well, no #$%, Brain Trust....

Point being, if one doesn't know how to, or outright REFUSES to, convert between metric and imperial....select the one supported by one's available facility and stick with it. Doesn't apply directly to MD's troubles, but may help in terms of general background info for others also trying to get a handle on things.

[joshs]

Since we're talking different zeros, one thing I never really considered until I took F2S's carbine class, was how different zeros affect trajectory when then gun in on it's side (SBU/rollover prone). Because the 200/300 yard/meter zeros have more "UP" elevation, when gravity is not acting against the "UP" direction (when the carbine is turned 90 degrees on its side), they tend to deviate from PoA much more quickly than a 100 yard/meter zero.

[rob_s]

Since we're talking different zeros, one thing I never really considered until I took F2S's carbine class, was how different zeros affect trajectory when then gun in on it's side (SBU/rollover prone). Because the 200/300 yard/meter zeros have more "UP" elevation, when gravity is not acting against the "UP" direction (when the carbine is turned 90 degrees on its side), they tend to deviate from PoA much more quickly than a 100 yard/meter zero.

Out of curiosity, was this demonstrated somehow, or discussed as theory ?

For quote some time I was planning on hitting the range with a bipod. It takes to the 9 o'clock rail to play with checking POI.

[joshs]

Out of curiosity, was this demonstrated somehow, or discussed as theory ?

Mostly discussed, but we shot on vertical 3x5 cards at 75 yards from rollover to demonstrate the lack of deviation from PoA using a 100 meter zero. F2S allowed people to keep a 50 meter zero, but everyone in the class I took zeroed at 100m so there was no example in class of the alternatives. I've since confirmed it on my own using a 300 yard zero. I know use 100 meter/yard zeros as long as the optic I'm using has some type holdover reticle feature. With irons or other optics I use 50 and then try to confirm the long intersection when able.

[iakdrago]

Don't want to beat a dead horse, a lot of great info in here that i have copied and pasted into a word document. I have a question on body position when zeroing. So far, i have been using the prone position for zeroing my Ar. When shooting of the bench, i do notice a POI shift when my body is upright, as opposed to horizontal. My best educated guess is that it has to do with a change in the distance of my eye to the rear sight between the two positions. Recently, I was invited to participate in a "woods walk" on a wooded property with targets set at various locations. In none of the "engagements" i found myself using the prone position, nor would it have been very viable (South East PA). Most of the time i was either A) Standing or B) Squating with using a tree as a barricade brace whenever possible. This led me to reconsider my "prone" method of zeroing my sights--should i be zeroing of the bench where my body is more vertical and the rifle orientation resembles that of a standing/squatting position?