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Thread: Basic trajectory curve--5.56

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    Basic trajectory curve--5.56

    Can someone help this poor novice out . Getting ready for sight-in of M4. Cannot seem to find easily understood 5.56 curves. Some one told me that zero at 25 meters will get a zero at about 250 meters, with a 16" barrel and a 55gr round. Need your expert instruction.

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    (this thread probably doesn't belong in the terminal ballistic forum)


    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 upwards towards 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 or 215 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 more 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.





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



    Courtesy of zrxc77





    M855 25 yard zero





    M855 25 meter zero





    M855 36 yard vs 100 yard zero






    M855 and M193 25 meter zero






    M855 and M193 50 yard zero from 16” barrel





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






    100 yard zero






    25 meter vs 50 yard zero, M855 from 16” barrel






    M855 300 meter zero






    M855 200 meter vs 300 meter zeros





    M855 36 yard vs 50 yard zeros




    continued
    Last edited by Molon; 10-03-10 at 21:44.
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    RIBZ

    Revised Improved Battlesight Zero

    The standard A2 rear sights on an AR-15/M16A2 were designed with elevation settings for 300 to 800 meters. The Santose Improved Battlesight Zero allows for an elevation setting of 50 yards/200 meters for one of the most all-around useful trajectories obtainable with the 5.56mm/223 Remington cartridge when fired from an AR-15.

    Neither of the above sighting schemes allow for an elevation setting giving you point of aim equals point of impact at 100 yards. Since 100 yard shooting ranges are some of the most commonly found ranges in the United States, it would be useful to have such a setting on our AR-15s. This can be achieved quite easily with nothing more than a 1/16” allen wrench. It’s really just a matter of taking the Improved Battlesight Zero one step further.

    There is a witness hole on the A2 rear sight base that gives you access to the index screw that you will need to loosen with the 1/16” allen wrench to make the necessary alteration to the elevation drum. The witness hole lines up with the index screw only when the elevation drum is set to 8/3. Set your elevation drum on the 8/3 setting and turn the screw counter-clockwise 3 to 4 complete turns. You don’t want to remove the screw. You want to loosen it enough to allow the elevation index wheel (the top part of the elevation drum with the numbers on it) and the elevation knob (the bottom part of the elevation drum) to separate and be able to move independently of each other.


























    Once you have loosened the index screw, use one hand to hold the allen wrench in place on the screw and with your other hand turn the elevation knob (the bottom part of the elevation drum) 4 clicks clockwise.* You should see the rear sight base moving up while you do this. The elevation index wheel should not move while you are doing this. (We really only need to turn the elevation knob 3 clicks clockwise, but by turning it 4 clicks we get a “buffer zone” that keeps the rear sight base from coming into contact with the lower receiver when the sight is moved to the lowest used setting. This gives you a more consistently repeatable sight setting.)

    Now that you have moved the elevation knob 4 clicks clockwise, taking care to make sure the elevation index wheel has not moved, firmly tighten the index screw. Starting with the elevation drum at the 8/3 setting, you should easily be able to turn the drum counter-clockwise 3 clicks and see the rear sight base moving down. (You should be able to turn the elevation drum one more click counter-clockwise for our “buffer zone.”) You are now ready to head to the shooting range.

    As I mentioned earlier, this sighting scheme is really just taking the Improved Battlesight Zero one step further. Starting with the elevation drum at the 8/3 setting, turn the elevation drum counter-clockwise 3 clicks (8/3, -3). This is your 100 yard point of aim equals point of impact setting. From a distance of 100 yards, zero the rifle with the rear sight at this setting (using the small sight aperture). Do not change this rear sight elevation setting while zeroing. Use the front sight post only to make changes in elevation while zeroing.

    The beauty of the RIBZ sighting scheme is that once you have zeroed the rifle with this method, you will have your 100 yard zero setting and the other settings of the elevation drum remain intact (within the limits of the coarse adjustments of the A2 sights). If you want to use the Improved Battlesight Zero, simply set the elevation drum to 8/3, -2 and you're good to go. To use the standard military 300 meter setting, just set the elevation drum to 8/3. You can also start the zeroing process by obtaining the Santose Improved Battlesight Zero at a distance of 50 yards with the rear sight on the 8/3, -2 setting. Obviously, slight variations will be encountered depending on the ammunition used and the actual amount of movement with each click of the various sights. The results will certainly be close enough “for government work.”

    When using this sighting method, you may find that once you have set your 100 yard zero, the top of the base of the front sight post is now slightly above the top of the well in the front sight base. If this happens, the “0.040” taller” front sight post from Bushmaster provides a quick and inexpensive fix.








    100 yard zero (8/3, -3)

















    Improved Battlesight Zero (8/3, -2)

















    Standard 300 meter zero (8/3)






    * E.T.A.

    All click references should be doubled when using detachable carry handle sights.


    IBZ (6/3, -4)







    continued
    Last edited by Molon; 10-03-10 at 21:45.
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    While the Revised Improved Battlesight Zero is certainly not perfect, here is an example of how well it does work at the distances typically encountered at the public KD ranges in my state. I zeroed my 20” Colt HBAR with A2 sights at 100 yards as described in this thread. Here is a pic of a 10-shot group from 100 yards.


    100 yards, (8/3, -3)












    Next, I packed up my gear and headed to the 50 yard line. By simply coming up one click on the elevation wheel (8/3, -2), I was in the 10-ring at 50 yards.

    50 yards, (8/3, -2)






    Lastly, I moved down to the 25 yard line. With this sighting scheme, setting the elevation wheel to (8/3, +3) [or (8/3, +2) depending on the rifle] gives me the necessary elevation for 25 yards. (Again, due to the course adjustment of the AR-15 sights and manufacturing tolerances non of these settings are perfect.) In fact, if you don't have access to a 100 yard range or you just want to “get on paper” before heading to the 100 yard range to refine the zero, you can zero at 25 yards using this setting.

    25 yards, (8/3, +3)





    Here is a down-loadable target in .pdf format that can be used for zeroing 20” uppers with A2 sights at 100 yards using the Revised Improved Battlesight Zero. The dashed vertical grey lines represent one click of the windage knob. The dashed horizontal lines represent one click of the FRONT SIGHT POST.

    100 yard zeroing target

    When printing, you need to have the “page scaling” option in Adobe Reader set to “none” in order for the target to print with the correct dimensions.







    continued
    Last edited by Molon; 10-03-10 at 21:45.
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    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. The unmarked aperture is used for normal firing situations, zeroing and with the elevation knob for target distances up to 800 meters. The unmarked aperture is used to establish the battlesight zero.” That’s pretty straightforward.

    Using a digital caliper, the diameter of the small aperture of the A2 rear sight measures approximately .070”. The large (0-2) aperture has a diameter of approximately .200”. No confusion there.

    To use the 0-2 large aperture, simply push forward and down on the small aperture portion of the sight leaf to snap the large, 0-2 aperture up into place. That couldn’t get much easier.

    The confusion surrounding the use of the 0-2 large aperture seems to stem from a misunderstanding of the amount of elevation change involved when changing from the small aperture to the large aperture. The small aperture is intended to be zeroed for a 300 meter zero using M855 fired from a 20” barreled M16A2. The 0-2 large aperture is intended to provide a 200 meter zero, when the small aperture has been properly zeroed with M855 from a 20” barreled M16A2. If you examine the trajectories of M855 fired from a 20” barreled M16A2 zeroed at 300 meters, and at 200 meters, you will see that the difference in elevation between these two trajectories (and hence the difference in elevation between the small and large apertures) is 2.5 MOA.




    To demonstrate this concept, I fired two 10-shot groups from a 20" Colt AR-15, using the standard A2 iron sights, from the bench at a distance of 50 yards using the small aperture. I then flipped the rear sight to the 0-2 large aperture and fired another two 10-shot groups (all groups were fired on separate targets.) The two sets of groups were overlayed on each other to form 20-shot composite groups using RSI Shooting Lab and analyzed to determine the statistical center of their points of impact.

    In a perfect world, the groups fired using the 0-2 large aperture would have the center of their points of impact 2.5 MOA (1.25” at 50 yards) below the center of the the points of impact of the groups fired using the small aperture. The 20-shot composite group formed from the 2 groups fired using the 0-2 large aperture had a center of point of impact 1.01” below the center of point of impact of the 20-shot composite group fired using the small aperture. 1.01” at 50 yards is 2.02 MOA; not exactly 2.5 MOA, but within a half minute of angle and well within my margin of error using the A2 iron sights. As a side note, the groups fired using the 0-2 large aperture were 144% larger than the groups fired using the small aperture.






    The shifts will definitely be greater with carbines due to the shorter sight radius. I’d like to thank Hallorann for sending me a link to the XS Sights (formerly Ashley Express) website where there is some information that may shed some more light on this subject. I have not experienced this myself and therefore cannot vouch for the validity of the information, but according to XS Sights there are three different A2 rear sight leafs on the market that have different elevation offsets; however, XS Sights does not state which manufacturers/models use which rear sight leafs. (My experience with the A2 sights has been strictly with Colt 20" models and they have all had the 2.5 MOA offset for the 0-2 large aperture, and yes, I drank the Kool-Aid.)

    According to XS Sights:

    “Comparing some of the rear sights available today there appears to be 3 variations available. The 1st style (and oldest) has a .014 offset. This gives a 2.52 inch shift in point of impact at 100 yards between the two apertures. The 2nd style has a .017 offset, which shifts point of impact 3.06 inches at 100 yards. The 3rd style has a .024 offset giving a 4.36 inch shift in P.O.I.

    All of the points of impact shifts shown above are for the standard barrel AR-15/M-16 that has a sight radius of 20 inches. The shorter sight radius (14.5 inches) of the M4 carbine and weapons of similar configuration increase the amount of point of impact shift. The shifts for these shorter sight radius weapons are 3.48, 4.25, 5.96 inches respectively.

    The only reason for the various shifts would be for changes in the ammunition used by the military over the course of this weapons employment.”

    The last statement in the above quote seems rather odd to me, as M855 still is, and always has been the standard issue ammunition since the adoption of the M16A2. Also, I don't see any particular use for the 3.06 MOA and 4.36 MOA offsets for a 20" barrel. With the small aperture properly zeroed for 300 meters, the 3.06 MOA offsett will produce a 165 yard zero and with the 4.36 MOA offset, the bullet will never cross the line of sight.


    20" barrel with M855 zeroed for 300 meters





    .....
    Last edited by Molon; 09-30-10 at 15:36.
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    ...


    Who ARE you, and who has this much information?

    Molon = All-knowing?


    By the way, thanks. This is an abundance of good information which 90% of shooters need to read.
    Last edited by _rab_; 09-30-10 at 17:08.

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    5.56 Trajectory

    You are obviously a ballistic professor. I am still reading all of the data, but is clear that I will have more than enough to do the job with precision.

    Thanks so much for these valuable inputs
    Last edited by 55LAF; 09-30-10 at 19:50.

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    Molon, wonderful post. Actually it is almost enough for an entire website by itself, and great info.

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    Another Molon post that should be a sticky!
    www.hardwiredtacticalshooting.com

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    Talk about your asked and answered
    "Intelligence is not the ability to regurgitate information. It is the ability to make sound decisions on a consistent basis "--me

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    Another Molon post that should be a sticky!
    Wayne is dead on! Thanks for the empiracal proof here Molon. LTC Chuck Santose is legend both in Nam and in Internet lore - still the old AR-15L lives on! Was waiting for him to be brought in the mix.

    50m has the feel of what state side is all about. Even LAV, as mentioned earlier, would be first to mention there are good reasons for different zeros - "just know them and understand why!" His +/- was not to do a 25m and for longer range engagements aka Afgan...100-200 might be more appropriate.
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    Sticky Worthy

    Lots of good information and photos here....This is a STICKY WORTHY Post if I have ever seen one.
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    Sight adj.

    Thanks for the in depth info about sight adj. I am just finishing a a2 build and was wondering how to set the rear and front sights. NOW I KNOW!!!!!!! Thanks again for great info!!!!! Will post outcome when I get a chance to go to range. Just going back to work from incounter with skin cancer on back!!! I think it came from being in Gulf war!!!! enough for me!!!!!!!!!!!

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    I want to zero my BUIS on my carbine (16" barrel) for 50 yards. The BUIS is a GG&G A2 style with the double apertures. Which aperture do I use for my 50 yard zero?
    Last edited by LOBO; 10-05-13 at 06:53.

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    Quote Originally Posted by LOBO View Post
    I want to zero my BUIS on my carbine (16" barrel) for 50 yards. The BUIS is a GG&G A2 style with the double apertures. Which aperture do I use for my 50 yard zero?
    Personal preference really. I think you'd be more accurate with the small. I pretty much use the the small aperature for everything except low light & close distances.

    http://www.m4carbine.net/archive/index.php/t-23814.html

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    If that GG&G is similar to the one I have the only difference is actually the size of the aperture. (same plane) If it is a "primary" back up the small is probably better. If it is truly a back up (ala primary optic) then the large is faster in practice, although the difference is probably nearly unnoticeable.

    Joe
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