All about AR-15 sights: there's more to know about iron sights than you might imagine!
Front Sight Mounting
The old school way of mounting a front sight was with taper pins driven I in at the factory by gorillas, the way the military still fastens theirs. Unfortunately, this practice has a negative impact on accuracy. Anything done on the outside of any gun barrel can influence the internals, more so in hammer forged or button rifled barrels which do not cut out the grooves but simply swage metal to form the grooves. Military M16 barrels are button rifled and the same factories normally hammer forge machine gun barrels. When the taper pins are hard driven in place they tend to constrict the bores of the barrels slightly and bullets passing such constrictions are boogered twice rotating past each pin. Bullets twice deformed on different sides don't shoot accurately.
It's not difficult to remove some of this bore constriction, as described in "Improving Rack-Grade AR-15s" (December 2014.) Because of the justifiable bad reputation tapered pins have earned regarding accuracy degradation, some barrel manufacturers have made an effort to either improve or obfuscate (depending on your point of view) matters by changing to straight pins. Whether these really deform the internals of a gun bore less is something I have never attempted to quantify but I will say that they are highly suspect. The techniques in my previous article can be modified to work with straight pins. If you are selecting a barrel for serious target shooting I wouldn't use any barrel with a pinned front sight. These are normally offered by low bid, lower quality barrel manufacturers, starting with a poor quality barrel blank and risking worsened accuracy by pinning the sight in place. On the other hand, if you are building a "minute of tin can" blaster, save money by choosing such a barrel.
Another way of fastening front sights is what I call a "clamshell method." The bottoms of both barrel bands are split open and the four holes in the housing normally used with taper pins are drilled. On the ejection port side of the sight housing holes are tapped for two fairly heavy Allen screws. The bolt catch side of the housing has the holes countersunk to fit the heads of the Allen screws. When those two screws are tightened, the "clamshells" constrict, firmly capturing the barrel and holding the sight in place. I use a simple pointer gauge to approximately center the front sight post initially. There are other ways of doing this that take longer and don't necessarily produce any better results. The shooter will have to establish zeroes and gunsmiths should just get the gun on paper. An advantage to this clamshell design is slightly loosening the Allen screws allows the sight housing to be moved right or left to allow no-wind zero (where the bullet hits when conditions are dead calm) and mechanical zero (where the rear sight is dead center in the middle of its left to right travel) to coincide.
MAK Enterprises or Ray-Vin front sight adjustment gauges make this easy. Note, when initially locating any of these gauges on pre-ban barrels, insure there is a slight gap between the front of the bayonet lug and the clamp on the gauge. Contact between the two will compromise your adjustments. Once that setting has been accomplished and the Allen screws retightened the barrel bands and Allen screws need to be hit with Loctite 290 Wicking Grade to "weld" everything in place.
A third way is using set screws up through the bottom. Set screws can be placed in either of two places or doubled up for a more secure hold. All A2 sight housings already have a second hole pre-drilled that was used in turn to drill the hole in the housing that feeds gas up from the gas port in the barrel to the gas tube that is anchored inside the sight housing. That hole in the bottom of the rear barrel band can also be tapped for a set screw. If a sling swivel is attached it will have to be temporarily removed to get to the hole to tap and install the set screw. After everything is adjusted and Loctite 290 has been applied, replace the original sling swivel rivet and remount the swivel. National Match uppers normally have float tubes to remove sling pressure from the barrel and some of these tubes allow easy access to the rear barrel band pre-drilled hole. The 1 1/4" unit from Compass Lake is an example of user friendliness whereas the 1 3/8" float tubes from Northern Competition and others requires machining out a "U" shape on the front of the sling swivel stud. Purchase float tubes accordingly.
Fourth is using set screws from the sides. This is the current flavor of the week in building National Match rifles. If you are working with a sight housing that was previously held on with pins, tap the holes to 8-32 and use four Allen screws (3/16" length will fit flush when screwed in, longer 1/4" screws protrude a bit. I prefer the latter.) You'll eventually find a customer who has boogered the hole for the Allen wrench and used Red Loctite to further make your life miserable. Some of these will require being drilled out. Using longer Allen screws from the start may protrude enough to allow getting a pointed vice grip on it for removal if stubborn. It doesn't work all the time but works often enough to make it worth considering. When using the four set screw approach, modify the , barrel by machining flats for the set screws to bottom out against. CLE (Compass Lake Engineering, compasslake.com) normally has these four set screw units in stock in both pre and post ban and various heights that will be described further along in the article. They can also machine the mounting flats in your barrel if you are not comfortable doing the work yourself.
Finally, there is the hybrid approach involving tapping two set screws in the forward barrel band from the sides (still requiring modification of the barrel to cut the flats) and then one set screw up through the existing hole in the bottom of the rear band. As previously mentioned, some float tubes will make this difficult and may require minor machining of the tube to accommodate access to the set screw. If you are going to do this I recommend using the same threads on all the Allen screws to make it a little more user friendly for the customer. White Oak Armament normally has three set screw hybrids in stock in both pre and post ban and in the different heights that will be described later.
Those who use the hybrid approach have their reasons. The first theory has to do with the tiny bit of gas that we see evidence of escaping around the top of the rear barrel band. They fear that losing this tiny amount of gas will either slow down bullet acceleration or result in malfunctions. The rear set screw tends to pull the top of the rear barrel band down very tightly with the top of the barrel and will help seal off gas escape. While this may make a few people feel better it will do no good. Both Derrick Martin of Accuracy Speaks and I have independently tested the complete shut off all gas at the port and not found a bit of velocity increase. Derrick reported his experiments in The Complete Guide to AR-15 Accuracy and I have reported my similar findings here previously.
The other theory for the hybrid approach has to do with the pressures on the side mounted set screws applied to the barrel. Nobody I know of uses torque wrenches when tightening these four set screws from the sides. It is not hard to imagine these unequal pressures may bend the barrel or influence harmonics. While evaluating some post ban barrels I tested this theory on the machine rest for grouping. I started with a barrel and a four-set sight on it and deliberately induced differences in Allen screw pressures. After shooting groups with known lots of ammo I unscrewed the side set screws and replaced the sight housing with a hybrid unit with two side screws on the front barrel band and one through the bottom of the rear band. I deliberately tried to apply equal torque to each of the side set screws up front and refired the same ammunition. All the guns were solid sub-minute and I saw no difference between the four set screw or hybrid housings.
I have seen gunsmiths using five set screws, four on either side as explained above plus one through the existing hole in the bottom of the rear barrel band. Once no-wind and mechanical zero are the same, the barrel bands and set screws need Loctite 290. Don't rely 100% on things holding and occasionally verify the set screws aren't backing out. Having the five set screws just adds one more layer of insurance against the appearance of Murphy, so I am not opposed to them.
Front Sight Windage
Most windage adjustments are done on the rear sight and front sight elevation is adjusted only during initial zeroing. Precision shooters prefer the mechanical and no wind zeroes to be the same and the methods described above (except for the old school approach) allow left to right movement after loosening the appropriate Allen screws.
Start with the rear sight set to mechanical zero and fire a group. Note the left to right distance and direction from target center and convert to minutes of angle. Clamp a sight gauge to the barrel with the stem of the dial indicator firmly pushing against the sight housing in line with the top of the sight post and set it to zero. Move the front sight housing in the same direction as the group from target center, the opposite direction you'd move a rear sight. For example, moving the front sight to the right causes the bullet impact to track left. How far? On a Service Rifle (standard AR-15 sight radius) one minute of angle is 0.006" of sight movement. Most of the gauges are calibrated in thousandths. Once you have applied the correction per the math above, clamp down the sight set screws, remove the gauge, and fire a second 4 group. Clear the rifle and check if additional fine tuning is needed. You shouldn't need to fire more than a few groups to get it right. Set it in place with Loctite on the barrel bands and set screws when done.
If your sight housing has set screws on the sides, loosen the set screws on the rifle's bolt catch side to move the sight left and tighten the set screws on the ejection port side. A tiny little movement in the set screws will move the sight (per the gauge) a long way so use restraint until you get the feel of things. On the hybrid sights that have bottom set screws, loosen them before attempting to make adjustments to the side mounted screws or you will be applying pressure but the sight won't be moving. After making the appropriate adjustment to the left or right with the front set screws, retighten the bottom set screw.
Pinned Sight Windage
Fortunately, most pinned sights come fairly well centered. These are not normally match quality rifles anyway but you may encounter one that needs corrective action. Determine how far off and the direction of the error. Derrick Martin suggests removing the front sight housing and sling swivel, then locating the gas port hole on the bottom of the rear barrel band. Drill it out, tap it to 10-32, and install an Allen screw short enough that it will clear the new sling swivel rivet when screwed all the way in. Or save the drilling step and just tap it to 832, but the fatter 10-32 set screw gives more screw-to-barrel contact surface to help hold things as we proceed. Put everything back together including driving the pins back in their original position. Screw the 10-32 set screw down tight. Carefully drift the pins back out and install the gauge. Loosen the set screw just enough to allow moving the sight base in the correct direction and called-for amount. Screw down the set screw tightly and secure with Loctite 290 on the barrel bands and set screw. After setting up overnight, dress the holes so the pins can be put back in place. Use either a drill or reamer of the correct size to match straight pins or a Number "0" taper pin reamer if using tapered pins. Once you establish a good fit, Loctite the pins as well. Put the sling swivel back on using a new rivet and you should be in business.
Hopefully, everything so far has been fairly straight forward. Unfortunately the waters will muddy somewhat with the next sections due to lack of uniformity by some manufacturers of certain parts. While I apologize in advance, please remember I am just the messenger and I am trying to make some complex topics as understandable as possible.
Afraid Of Heights?
You may not think you have acrophobia now but after reading this section you may be forced to reconsider. As near as I have been able to piece back together, life was good when all we had was the M16A2 with the standard front sight housing being 1.910". When Colt introduced the flat top and detachable carry handle things started to get mucked up a bit. It seems the rear sight aperture on the Colt carry handle sight ended up being a little above that of a standard A2 rear sight aperture when both were completely down. Raising the rear sight of any AR by clicking clockwise moves the point of impact up. A taller rear sight does likewise and front sight changes move POIs opposite, so Colt released a taller front sight housing to "level out" the problem with the taller rear sight and get the zero issue back under control. These taller sight bases sometimes have a raised "F" on them (for "flat top") but I have seen plenty that weren't marked. These sights run around 1.960" to 1.970". Bravo Company USA (bravocompanyusa.com) offers "F" sight bases with no holes drilled for pins or set screws.
This deviation from standard is a wild card and was somewhat inconvenient but still manageable if gunsmiths paid attention. Other manufacturers of flat tops are not required to follow Colt's example or dimensions. Some didn't and receiver profile heights vary between manufacturers. Detachable carry handle dimensions also varied and some don't match Colt units. I suppose it was preordained with all these changes and mismatches someone would come out with a sight tower mid way between the tall F-type and the original A2.1 call it a "tweener" measuring 1.940-1.950". To further complicate matters, shooters look at and line up sights differently and stabilize the gun in various ways. Ammunition loads are also different. To top it off, barrels also mount in their receivers differently resulting in gun-to-gun impact variations even with the same sights and fired by the same shooter.
What's a poor gunsmith to do? First, it may not be quite as bleak as I have painted things. The front and rear sights both adjust for elevation so some mismatch can occur and often not even be noticed. I normally use the tall F-type housing on all flat tops unless I have a reason not to. I would also use a lower sight base on a flat top if I know the owner was ever planning to shoot long range 1000 yard events because lowering the front sight raises the point of bullet impact and a bullet drops a lot at 1,000 yards. Shorter distance A2s get low or middle heights.
Sometimes an A2 and a customer will just mismatch and can end up with a shooter having to raise his front sight post way up out of the well. There are two potential cures. The most obvious is replacing the existing housing with a taller unit. This causes the customer to lose his zero and the firearm normally needs to come back to the gunsmith's shop to do the swap. A faster and easier way is to substitute a taller front sight post. There are three sources of these for 0.072" wide battle sights. DPMS makes a 0.030" taller post (Brownells 231-000-128), Bushmaster makes a 0.040" taller post (DPMS #F1003228) and Compass Lake Engineering makes one 0.060" taller. For more narrow 0.062" or 0.052" posts the only source I know of is Compass Lake. Tall posts let most customers swap them without returning the firearm to the gunsmith. With a caliper, determine the height of his existing sight post and duplicate that with the replacement to keep the same zero. The raised flange of the sight post will be down below the flat shelf and proper support will be given to the post so it doesn't move around.
You will also run into situations calling for shorter sight posts and I make it a habit to carry a modest stock of both tall and short posts in all three sight widths. My short posts are from Northern Competition and they are 0.050" shorter.
Most of the fat 0.072" battle sight posts are the same on all four surfaces, allowing one click increments of 11/4, just as Mother Nature intended. The narrower 0.062" and 0.052" versions are cut differently. They will be the stated width on the surface facing the shooter and taper down on the side opposite the shooter's eye. Likewise, they will often be taller on the surface facing the shooter and taper down toward the muzzle from the top. This combination is supposed to offer a sharper sight picture and allow one to better focus on the front sight. Adjustment wise, unfortunately, it complicates matters a bit. Instead of using every one of the four click notches, adjustments have to be in full five MOA revolutions. It helps me to scribe a white line on my sight adjustment tool to aid in insuring I've gone a full 360 degree turn when adjusting these sights.
Also handy here is a penetrating oil like Kroil. You'll likely get a customer that neglected his firearm. Perhaps he got caught in the rain and didn't dry the gun before storing it in a safe. The front sight plunger can end up rusting to the housing, making sight adjustments impossible. Sometimes these will be so bad that they will have to be drilled out. Prior to resorting to such drastic measures try soaking with Kroil overnight. If you are able to remove the plunger you may need to dress the hole in the sight housing if there is a lot of rust. The plunger measures about 0.167" and a drill about 0.1695" should do the trick. Some brand new housings will come from the factory with burrs and will need dressing before they will accept the plunger as well.
A decade ago it was popular to eliminate movement of the front sight post with a little set screw protruding down out of the female threaded hole for the male adjustment threads of the sight stem. That female thread doesn't necessarily go all the way down from the top. The male threads on the bottom stem of the post aren't that long so there was no reason to tap the entire length of the hole with female threads. Gunsmiths would start a tap (8-36) at the top of the threads, follow them down, and continue to cut new threads clear to the bottom. Then, with a very short set screw of the appropriate thread, run it up from the bottom with a cut off Allen wrench until it hit the bottom of the sight post stem. That would create a jamming effect and remove any possible movement in the sight post from vibration, recoil, and mishap. Some even got carried away and applied Loctite, thus insuring that if the sight had to be moved the process would be as difficult as humanly possible! Guess what? When you shoot 1,000 yards away you must adjust the front post and quite smartly at that! Just discovering that your well meaning gunsmith has a small set screw Loctited in place preventing your post from being screwed down it is guaranteed to send a shot of adrenaline through you that won't wear off before being called upon to fire 1000-yard match. Our team of Army Reserve Service Rifle shooters only had one or two using set screws. The rest wisely backed theirs out. I finally quit proactively even putting the set screws in there and only put then upon request and I had very few requests. Try wiggling a sight post on an AR-15 and it will likely have no front post movement. Gunsmiths going to all that needless work a decade ago was an example of what my friend, the late Jeff Cooper, would have called "The ideal solution to a nonexistent problem."
Be sure to protect your sights. Sight protectors made of hard plastic in clamshell shapes for both the front and back are worth their weight in gold! They keep the dirt out and protect against all but the most grievous damage. Any time you aren't shooting you should have these in place.
Zeroes And The "Dead Zone"
In most applications of the black rifle we set the height of the front post once during the initial zero phase and never touch it again. All adjustments for distance are accomplished on the rear sight. Thousand yard shooting is different, however. You simply can't get enough elevation adjustment out of a regular A2 rear sight and need to use the front sight and to reach out about 6/10 of a mile we have to lower the front post significantly.
The majority of National Match shooters base their 1,000 yard zero off of a known 600 yard zero. Most shooters fall into two camps. The first (screws the front post down three complete revolutions for a total of 15 minutes, then comes up approximately 40 clicks on the rear sight (10 minutes for 1/4 minute match sights.) The exact number of clicks on the rear sight is extremely variable depending on the shooter's ammo, his sight picture, what hold he uses (6 o'clock, flat tire, center mass, etc.) You can give or take at least a couple of minutes from what I quoted. The 40 clicks on the rear sight will get the shooter's head up pretty high on the stock and may compromise cheek weld some. It will allow plenty of post to stick up above the shelf of the front sight, however, which some shooters like.
The second camp goes down another full revolution (four total) on the front sight (20 minutes) and comes up about 20 clicks on the rear sight. There will obviously be less of the front sight post showing above the housing but the cheek to stock weld should be better because the rear sight isn't jacked up so high. It is because of the second camp that I choose to outfit my 1000-yard rifles with the lowest front sight housings, allowing more of the post to show above the housing. In order to keep track of these downward front sight revolutions I like a white vertical paint mark on the front sight adjustment tool. Also remember to run the front sight post back up to its normal position.
Both of the sight adjustments will arrive at the same 1,000 yd. Point Of Impact (POI), just in two different ways. Which is better? It depends on the dead zone. Many common and popular rear sights on the market have a dead zone at the top of their range, about 12 clicks clear at the top. The number of clicks is quite variable. When you jack that rear sight up so high into that dead zone nothing happens. Continued clicking feels normal but the sight doesn't move! Test for this by attaching a sight gauge set for elevation tracking. Run the rear sight up to the top and click back and forth and see if it is responsive. If not, work up and down until you find where the clicks actually start to register. I used to go down one or two additional clicks from that point and paint a red vertical line on the elevation wheel as a reminder to the shooter. It is common for Compass Lake Engineering, White Oak, and Rock River rear sights to have dead zones at the top. I don't know how many other brands have this because I only use the three mentioned companies due to their normal superior quality otherwise. Northern Competition markets a 1/4 x 1/4 MOA rear sight claiming it's designed to avoid a dead zone but admit that it also has just a few less usable clicks than the others so I don't see that as a solution either.
Since so many sights out there have dead zones the right way to deal with it is shooter management. Begin with getting the right base zero established at the closest distance the shooter will ever fire. All NMC shooters fire Standing and Sitting Rapid strings at 200 yards, however, many of them practice at 100 yards on reduced targets. Those folks need to establish their base zero at 100 yards to accommodate this. To establish the base zero the shooter begins with the rear sight down as far as it will go, then come up about three clicks to allow lighting variances and other factors that influence elevation. Some shooters will need more than this. Then use the front sight post to finish elevation zeroing at the shortest distance. Elevation is then adjusted from here up to 600 yards with the rear sight and recorded. The proper base zero coupled with going down four complete revolutions on the front post and up about 20 clicks on the rear sight from the 600 yard setting should keep shooters safely away from the dead zone. As long as the shooter establishes a base zero using the instructions above, worries about the dead zone only apply to 1,000 yards. At 600 yards the sight should never be up high enough to encounter this.
The use of 20" barreled flat tops with carry handles in NMC shooting is becoming more popular but is it possible to use them out to 1,000 yards? Depends! It is common to use the tall "F" front sight housing with flat tops and that's not good for 1,000 yards. Also, carry handle rear sights don't have as much upward adjustment as regular A2 housings. I measured the lengths of the elevation thread stems on both and found a difference of 0.068" which is the same as 45 quarter minute elevation clicks. Northern Competition has worked out a solution for the rear elevation problem on carry handles by substituting the longer sight base of the normal A2. When screwed clear down this will result in the thread stem protruding out the bottom of the carry handle which would prevent it being mounted on a flat top. They simply locate and drill a hole in the top of the flat top receiver to allow the excess sight stem to go down into it. If that somehow cosmetically offends your sensibilities (it shouldn't because the hole is covered by the carry handle and is not visible) you could always have two carry handles, one set up with the standard rear sight base and a second with the tall base. The second carry handle would be reserved strictly for 1,000 yards where the rear sight is going to be up anyway. In that option the bore hole in the top of the flat top receiver isn't needed. The trade off is saving machining time to bore the hole versus the added cost of a second carry handle. The 1000-yard carry handle could stay zeroed for that distance all the time requiring only the appropriate front sight adjustment on days when the shooter desires to shoot the longer distance.
"Where there is a will there is a way" is a good note to close this article. You now know the sight tower and post height options, and how and when to apply them either in helping customers get zeroes or when building uppers from scratch. Keep a modest stock of the sight housing options and different height posts and you should be able to cope with the challenges using the tips in this article.
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|Title Annotation:||TECH TOPOICS|
|Date:||Aug 1, 2015|
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