I thought I'd put it up here as a straw man and let you guys who know more on the subject and are interested throw rocks at it and knock it down - and hopefully we can make it better. This is not a thread pitting iron sights vs. optics, the real issue is ballistics and sighting in. We all choose what's best for us but still all share the problem of getting POA to equal POI at various distances.
Would appreciate any comments and I hope this thread and its replies end up shedding more light on the subject! Thanks!
Zeroing the M4 and AR-15:
Taking Full Advantage of Carry Handle Sights
Getting your AR-15 on target at various distances can be a mystery since there are many zeroing schemes in use and many variations of the rifle. Then add in different barrel lengths which perform different ballistically, the wide range of ammo, and just what distance is best to zero for in the first place. Also, developing fundamental shooting and sighting skills takes dedication, effort and good eyesight. On top of that there is so much misinformation it’s hard to wade through all the crap to find what works best for you. Even manuals that come with rifles often are wrong! No wonder many AR owners give up on iron sights and go to optics thinking that will solve the problem, but technology also has difficulty solving all the problems of staying close to the bullseye at multiple distances.
Another problem is there are several versions of the carry handle rear sight that operate differently yet formal instruction tends to be given for one type, usually the A2 fixed unit found on the M16A2 and AR-15s based on this 20" rifle. However, most ARs sold today are M4 flat top carbines with a 14.5" or 16" barrel and often come with the A4 (M16A4) removable carry handle. When I bought an M4 style carbine and A4 (usually called A3) carry handle, I found it surprising how difficult it was to find correct information on operating the basic sighting system of such a popular choice. And that’s a shame since, even if it soon gets replaced with an optic, the A4 carry handle is a flexible sighting device (especially with modifications described below) that should be better understood. Good optics have important advantages such as quick target acquisition but if you have an unused carry handle it’s well worth investing some time and ammo to discover how useful it really can be. Also chopped carry handles work well in a back up role with many optics, especially red dot sights.
The path to unraveling the mystery begins with understanding the trajectory of the 5.56mm bullet in flight from the rifle's muzzle until it impacts a target. When you aim the rifle sights at a target, the line of sight is a straight line through the rear sight aperture, over the front sight post then straight out to the target. This is known as Point of Aim (POA). However much like a 50-yard football pass, the bullet travels in an arc, not a straight line, and wherever it hits is known as Point of Impact (POI). The trick is to get the straight line POA to equal the POI on that arc - and have it happen dead center on your target. At the muzzle the bullet is about 2.5" below POA because the sights are about that much higher than the centerline of the barrel. But the bullet soon rises above POA then downrange it falls back below POA. Depending on the rifle's initial angle of elevation you can expect a bullet to crossover the POA twice, once around 25-45 meters as it rises above POA and again around 200-300 meters out as it falls back to earth. Ballistics of 5.56mm bullets are well understood and the arc or trajectory is consistent if the rifle and ammunition and rifleman are all up to par. Understanding the bullet trajectory allows you to zero in the rifle at the closer crossover point (where POA = POI) and expect that the second crossover point also will be in zero, again POA = POI. The advantage is you zero at a shorter range, even a pistol range, with confidence that your long range zero will be true as well. Still, check that long range crossover on a day when the wind is calm.
Two popular zeroing methodologies are covered here - the US Armed Forces 300-Meter Battlesight Zero and the Santose Improved Battlesight Zero. We'll start with the 300-meter Battlesight Zero first because it's quite easy and is tailored for the M16/AR-15 carry handle with no esoteric adjustments. This is the method that's in general use by the US military while the Santose method hasn't been officially endorsed by the US Armed Forces at large but is gaining ground in and out of the military.
Battlesight Zero (BZ) is a handy method to get a military 300 meter zero at a 25 meter range (easier to find than a 300 meter range) because of the fortunate coincidence that the M4's crossovers are 25 and 300 meters. Get a good zero for 25 and you should be good to go for 300 meters as well. (See the second paragraph below for how the 20" rifle is zeroed at 25 meters.) BZ is described in detail in the M16 Operators Manual TM 9-1005-319-10 but let’s cut to the chase here.
Flip up the small aperture on the rear peep sight so it is used for sighting. Then set the M4 carry handle elevation knob to 6/3. Using 62 grain M855 FMJ and the targets linked below, fire groups of three shots and check the target after each group. To adjust bullet strike or POI left or right, use the windage knob on the right side of the rear sight. To adjust for elevation, leave the rear elevation at 6/3 and adjust the front sight post up or down. When shot groups are satisfactory at 25 meters, the M4 is successfully zeroed. To hit targets beyond 300 meters, turn the elevation knob clockwise to 4 for 400 meters, 5 for 500 and 6/3 (one full rotation) for 600. Return it down to 6/3 for normal range.
An AR-15 with a 20" barrel is battlesight zeroed differently because its first crossover is farther out, about 31 meters. Because of that, the A4 elevation knob is turned two clicks clockwise to the small 'z' mark before zeroing at 25 meters. After zeroing, move it back to 6/3 and the rifle is zeroed for 300 meters. For rifles with the A2 fixed carry handle marked 8/3, the knob has coarser adjustments where one click equals two on the 6/3 sight. So to zero a 20" rifle with A2 sights at 25 meters click the elevation knob one click clockwise instead of two. After zeroing move it back to 8/3 and the rifle is zeroed for 300 meters. AR-15 rifles with M16 or M16A1 sights do not have an elevation knob but can be "zeroed" about an inch low at 25 meters to achieve a 300 meter zero.
Links for more information:
http://www.armystudy...6a2-rifle.shtml Battlesight Zeroing the M16A2 rifle (for the M4, remember to zero at 6/3)
http://ohmr.ohio.gov.../ohmr2444-b.pdf Download Free M4 25/300 meter zeroing targets
http://www.bobdbob.c...a2-25m-zero.pdf Download Free M16 25/300 meter targets
http://www.usaac.arm...M16-M4 data.pdf US Army Rifle Marksmanship PDF with bullet path tables and graphs of the M4 carbine and M16 rifle
Looking at the 300m tables and graphs in the US Army Rifle Marksmanship PDF it's clear that between 50 and 275 meters the M4 and M16 shoot quite high - up to 7" high at 175 meters. And since the elevation knob is already cranked all the way down, there's no easy way to get the sight lower. The military does train holding the sights under the target and flipping the sight to the larger aperture that's marked 0-2, both of which bring down the point of impact. Unfortunately both techniques work with varying degrees of success depending mainly on the skill of the rifleman.
The deficiencies of the 300m battlesight zero at midranges lead us to the second zeroing scheme The Santose Improved Battlesight Zero (IBZ) which takes full advantage of the flat shooting 5.56 when zeroed at 50y/200m. Popularized by Lieutenant Colonel Chuck Santose, US Army (ret.), this ingenious technique involves modifying the A4 carry handle so the elevation knob bottoms out 4 clicks below the 6/3 setting. (The A2 fixed carry handle is set 2 clicks below 8/3.) To perform the modification, use a 1/16" Allen wrench to loosen a set screw so the elevation knob's top and bottom move independently. Turn the bottom ring four clicks clockwise making sure the sight post rises but 6/3 remains stationary and aligned with the index mark above it. Tighten the set screw firmly and turn the knob counterclockwise making sure that you can go four clicks below 6/3 (6/3 -4) then zero at 50 yards. Or click up to 6/3 and zero at 25 meters as described above in the BZ method. Since the rear sight has been raised, the front sight must be raised the same amount when zeroed. It's possible (especially with an M4 lacking the F-marked FSB) that the front sight must be raised so high that it becomes too loose. In that case Bushmaster's 0.040" taller sight post should solve the problem and only costs about $6. http://www.bushmaste...ucts.asp?cat=21
Once the M4 is zeroed in you can click below 6/3 four clicks to have the IBZ ranging scheme thatís within an outstanding 2.5" of line of sight from 0 to 230 meters! Thatís worth repeating - POI is within 2.5" of POA from 0-230 meters! (Of course that assumes you do your part of the job.) There is an enhanced version of the IBZ known as the Revised Improved Battlesight Zero (RIBZ) where the elevation knob is modified 6 clicks below 6/3, putting the M4 or M16A4 dead on at 100 yards. (For A2 sights go 3 clicks below 8/3.) In addition to 6/3 -6 for 100 and 150 yards, click to 6/3 -4 for the flat trajectory of the IBZ with zeroes at 50 yards and 200 meters, or 6/3 for standard BZ 25/300 meter zeroes including A4 ranging for 400, 500 and 600 meters. (My M4 did need a taller front sight post which had to be screwed deep in the well so I set the rear sight to bottom out 10 clicks below 6/3 which brought up both sights to a suitable level.) The RIBZ allows a quick click of the elevation knob for zeroes at most standard ranges found in the US: 25, 50, 100 and 150 yards plus military ranges too. Don't expect precision zeroing at every distance but it is astonishing how well this works. Definitely read the links provided for more information on these modifications. At first it looks daunting but actually is very easy to accomplish.
With the RIBZ modification an M4 should be on target if the elevation knob is set to the following:
Distance/Setting (y =yards/m=meters; up to 100 meters the difference doesn't greatly affect POI)
50y: 6/3 -4 (for the 20" A4 rifle; the M4 carbine will zero at 41 meters/45 yards)
100y: 6/3 -6
150y: 6/3 -6
200m: 6/3 -4
600m: 6/3 (after one full rotation upward)
http://grburnett.us/...lesightZero.pdf the Improved Battlesight Zero
http://www.m4carbine...thread.php?t=22 more about the Improved Battlesight Zero
http://www.ar15.com/...&...18&t=328143 more about the RIBZ
Consider the advantages of zeroing your BUIS and optics or even the old A1 carry handle with the Santose Improved Battlesight Zero. Though you won't have the handy elevation knob to make quick changes, you'll still gain the IBZ's flat trajectory from 0 to 230 meters. Think about that, set it and forget it - POI is within 2.5" of POA from 0 to 230 meters! Just zero in at the first crossover for your barrel length using the US Army Rifle Marksmanship PDF (200m table) or zero one inch low at 25 meters. (Remember Bushmaster's tall sight post if your front post gets too loose when you raise it.)
Bushmaster carry handle adjusted so it will go 10 clicks below 6/3
ArmaLite gas block front sight base with the 0.040" taller Bushmaster post
Edited by Quentin, Dec. 08 2009 - 04:21 PM.