[Gus Fisher]

Part I – Checking Rear Sight Assemblies

Folks, there is a good bit of “Ye Olde Art and Mysterie” about working on rear sights than many folks imagine and it is not an exact science nor a simple matter of replacing parts to get them right. What makes it even more difficult is that no commercial receiver is totally correct in the rear sight area. Even when you have a correctly milled rear sight area as on a REAL G.I. M14 or M1 Garand, there are still enough tolerance differences or tolerance stack up that can cause you problems. Also, there is no way a fix for one Rear Sight (RS) will automatically fix a problem with a RS on a different receiver.

The first thing I do when inspecting a real M14, M1 Garand or any commercial M14 rifle is to grab my pair of Brownell’s Magazine Tube/Cap Pliers. These have hard rubber like pads that will hold the pinion drum securely without damaging it. You could also wrap thin leather around the pinion drum and grab them with slip joint pliers. Then I use a screwdriver with a tip that matches the slot in the nut in the center of the pinion and try to tighten it. Usually, I find these nuts somewhat to very loose and that’s bad as they need to be tight. Sometimes all you have to do to fix a problem rear sight is to tighten this nut. I’ve also been informed the Brownell’s pliers were actually some kind of automotive pliers, but I’m not sure as I’m not an auto mechanic. Anyway, here’s a link showing these pliers.

http://www.brownells.com/.aspx/pid=13787/Product/MAGAZINE_TUBE_CAP_PLIERS

Once you know the nut in the pinion is tight, it is time to check the windage knob to see if you can turn the knob. If it is too tight or too loose, then the nut in the windage knob may not be tightened correctly. What I have found to be the “sweet spot” for most sights is that when you tighten this nut, the first time it JUMPS into position is most often the correct amount of tightness. I sometimes go one more half turn when the elevation pinion has a worn triangular projection or the spring tension in the pinion is worn. If that causes the windage knob to be too tight, then you need to back off the nut to the first setting. SPECIAL NOTE: I have used this extra click of tightness on windage knobs that make it a little difficult to turn the windage knob to save the person from having to buy a new pinion. If WHILE you push inwards on the nut in the elevation pinion and at the same time then can move the windage knob, that may save you from having to buy a new pinion for a while. It isn’t as convenient as just turning the windage knob, but it saves you rather big bucks on a new pinion.

The next thing I do is run the aperture up about 10 clicks to see the clicks are solid and consistent. Then I leave the aperture there. I use my thumb to press downwards and forwards on the aperture to see if the aperture will slip and slide downward. Now A CAUTION HERE: On even the most perfectly fitted and working RS assembly, if you use enough pressure, you CAN AND WILL force the aperture down. That is NOT a valid test. If the aperture slips with very little pressure, you can try tightening the windage knob one more half turn. If that doesn’t fix it, you may be able to fix the problem with a tighter fitting rear sight cover, but you may also need a new elevation pinion. If that extra half turn on the windage knob does fix it and if now you can not move the windage knob, again you may need either a different rear sight cover or a new elevation pinion. On NM RS apertures, I run the aperture up to 30 clicks and try it again as NM shooters will often use that much elevation.

The next check is to turn the windage knob 12 clicks to the right and back to zero than 12 clicks to the left and back to zero. (24 clicks each way with NM ½ minute windage knobs.) This ensures the RS base moves correctly and comes back correctly according to the hash marks on the receiver. If the windage knob is too hard to turn or skips or catches, then a different RS cover is often in order OR you have problems inside the receiver.

The next thing I check is to push in on the right and then the left side of the RS base. You want the RS base to either not move or spring back when you release tension. This was not considered absolutely necessary on a G.I. rifle, but it makes for a more consistent rear sight adjustments. If it does not pass this test, usually you need a different RS cover.

There are additional things we check on NM RS assemblies, though that can go to a full book length to explain and really is too involved for anyone but an Armorer or Gunsmith.

 

[DaveH]

WOW. That's all info we can really use, right now - without beacoup expertise. Nice job.

 

[Gus Fisher]

Part II

Inspecting Rear Sight Parts

The first part to inspect is the elevation pinion. After tightening the nut, you want to look at the condition of the triangular projection on the inside of the drum. That engages the serrations on the left side of the receiver. If that triangular projection is rounded over or worn down a lot, you have to replace the pinion because it won’t give you good clicks of elevation. There are some people who are rewelding and recutting these projections, but the cost is now more than a replacement pinion.

You can't really tell by sight if the windage knob will work correctly unless it is obviously damaged. I’ve only run across a half dozen windage knobs that had the “C:” spring worn too much in over 35 years of working on these rifles.

On apertures the first thing to check is to see you have a good ROUND hole in it. If it is dinged or malformed, it is really better to replace it. Aperture sights work because the greatest amount of light coming through them is in the middle of the hole. If the hole is not round, that will throw your eye off even if you don’t realize it. I also prefer to use apertures that fit as tight as possible side to side and still move freely – even with standard G.I. sights. I measure the width of the apertures and the grooves in the RS base for them to ge the ones with the least amount of slop between these two parts. Now, they didn’t worry about that on G.I. M14’s or M1 Garands, so it is not absolutely necessary, it just makes the RS a little better. I know of no one who will sell you a different standard RS aperture by width, but if you go to Gun Shows and take a pair of dial calipers or micrometers, you may find a wider one than you now have.

You basically check the RS base for obvious damage or cracks. They are actually a very strong part and only very unusually do you find something wrong with them.

As to the RS cover, we have to remember it is a large spring. About the only thing you can look for is the condition of the indent that bears against the top of the RS aperture slide. You turn it upside down to check it. I prefer ones that are not gouged in the center of the “dimple” as that can cause problems when you move your windage knob side to side. You can’t look at them and tell if it will fit correctly, or if they are too tight or too loose. NOS RS covers CAN have too much tension for commercial M14 receivers due to the way those receivers are milled for the cover and base. That’s important to remember. I’ve seen NOS RS covers that froze up the RS assemblies HARD on commercial receivers. So it is best not to throw away a RS cover as many will fit correctly on some other rifle.

 

[Riflenut]

MGySgt Fisher,

Thank you for taking the time to share your expertise.

This series needs to be saved as a sticky for future reference, as it is something that needs to be checked on every rifle!

 

[Gus Fisher]

The next check is to turn the elevation pinion 12 clicks to the right and back to zero than 12 clicks to the left and back to zero. (24 clicks each way with NM ½ minute windage knobs.)

Gus, you meant 'windage knob' rather than 'elevation pinion'. Obviously just a pencigraphical error. dave

OOOPPPPSSSS.......... Thanks for figuring that out and posting it. I just changed it. Thanks for the excuse, but I missed it pure and simple.

 

[Gus Fisher]

Part III

Some of you have read that I was the Instructor of OJT’s (On The Job Trainee’s or Apprentices) and later on the NCOIC of the NM Rifle (M14) Rebuild Section at the Rifle Team Equipment Repair Shop at Quantico, VA while on active duty. I didn’t come from a metal working background when I came to the RTE shop for my one year OJT program, so I had to sweat some blood to catch up or work things out. (For example: It was NOT pretty watching me figure out how to hand file metal flat. Grin. But, when I figured out how to do it, I could teach it to others so they would better understand.) That actually made me a better Instructor as I could understand the difficulties my OJT’s were having and it caused me to look for ways to do things in an orderly approach.

I mention this because instead of using a “shotgun” approach to working on rear sights where I give examples of “If this is wrong – try this, that or something else,” I believe it is far better to go through the whole RS in a systematic approach. That way you can identify and correct things as you go and you will better understand how everything is supposed to work. However, I’m also sorry to say that sometimes you have to go back and change something when further along in the process you find something wrong. IOW, don’t expect that changing one or more things will automatically give you a good to great RS. You have to use patience to work RS’s and sometimes you just have to have the stubbornness of a Mule and not give up to get them right.

SPECIAL NOTE: Over the last 35 plus years of working on commercial receivers, I have learned the HARD WAY that the very first thing to do on a new/bare commercial receiver is to try to fit the RS. If the RS area of the receiver is screwed up badly, then the receiver goes back to the factory as long as there is a warranty on the receiver. On a few commercial receivers over the years, I built the rest of the rifle first and did the RS as one of the last things like we did on REAL G.I. M14’s and M1’s. Well, a few commercial receivers were so bad there that I had to strip the receiver down and send it back – thereby wasting a lot of building time on those receivers. So by doing the RS first, you may avoid some serious problems.

Trust me, I’ve learned it saves time to go through the following procedures when working on commercial receivers and will go through these techniques when I have a problem with the RS on a REAL G.I. M14 or M1 Garand.

OK, the first thing is to either start with a bare receiver or take the whole RS assembly off the receiver. Orientation of the receiver for this whole procedure will be the front of the receiver pointing away from you and in the upright position, the same way it would be on a rifle when you fire it. To disassemble a RS, you first loosen the windage nut until it just spins and you can pull the elevation pinion out from the left side of the receiver. Then unscrew the windage knob until it comes free of the RS base and pull it out the right side. Then take a screwdriver and force it in place under the bottom rear of the RS base and between the RS base and receiver. Pry up on the screwdriver until the RS Base and RS cover pop out as a unit. Pull the aperture up and back it out of the rear sight base. Then the Base and cover will come apart in your hands.

I bought a ¾” wide, triangular, Medium India stone years ago to use for RS work. The reason I bought the triangular or “Three Square Stone” as it is also called, was it was cheaper than a square stone, not because the triangular shape is better. So if you find a square stone instead, that’s fine. What I do is lay the stone all the way up into the RS base area on the receiver. I keep it flat and go forward and back and side to side for a few strokes each way. I’m not trying to remove any metal. What I’m doing is to polish the surface the RS base moves over and to see how flat and even the surface is. This because the RS base is going to move side to side over this area and you want that surface to be pretty flat. Even on G.I. receivers, you will see milling marks there and that’s no problem. What we don’t want is high spots in the very front of the milled out area as that can/will cause the aperture to tighten up or freeze up too much and we don’t want the very rear of the milled out cut to be high as that could cause the RS base not to travel smoothly. Even on G.I. receivers, this surface will not be and doesn’t have to be perfectly flat and smooth.

Now, I can’t tell you how many times over the years I’ve seen problems with this surface on commercial receivers. I’ve seen high spots on the front and rear and high spots to the left or right sides. In worse case scenarios, I’ve had to use a stone or diamond file to flatten the high spots and get the surface more even. You can use the ¾” wide stone to level things up, but if it takes more than you can flatten with 20 or 30 strokes of a stone – it is time to return the receiver/rifle to the factory or take it to a real M1/M14 Armorer. I can not and do not recommend the average person use a diamond file to flatten this area as you can ruin a receiver.

The next step is to put JUST the elevation pinion and windage knob in place through the holes of the receiver “ears.” I’ve actually seen receivers over the years where the holes for these parts were too small or off angle. Usually such problems can be fixed by a knowledgeable Armorer or gunsmith enough where the RS will work correctly. Tighten the windage knob until the nut JUMPS in place as that is the sweet spot. What we are checking is the serviceability of your elevation pinion and windage knob. If you get good, noticeable and even audible clicks when you turn the elevation pinion, that’s what we want. If you don’t, then it’s time to look at the triangular projection of the elevation pinion and the receiver serrations. If the pinion and windage knob are good, you can NOT fix the receiver serrations. You may have an elevation pinion where the spring inside is worn too much and a substitute pinion will work. If that doesn’t work, then try a different windage knob. It those don’t work, that would cause you to send the receiver/rifle back to the factory OR you can use a set of WWII M1 Garand Lock Bar sights on the rifle. Over the years, I’ve seen about a dozen commercial receivers where you had to use Lock Bar sights to get a useable RS. That was the only choice when the factory warranty was only good for a year or no factory warranty for whatever reason. NOTE: There is no warranty on the Chinese receivers, so using a set of WWII Lock Bar sights may save your bacon with some of them.

OK, with the elevation pinion and windage knob working or replaced, it is time to go on to the next thing. This post has gone on long enough, so I will write more in the next post. Stay tuned.

 

[Gus Fisher]

OOps, a P.S. to my last post.

When the holes in the receiver for the elevation pinion and windage knob are too small or "cattywompous," you can usually fix them with a FINE sanding cartridge roll in a dremel tool used very carefully. What I do is put a layer of Dykem blue all around the holes and see where they are binding - by where the blue dye is worn off. A little polishing with the fine cartridge roll or a cratex point is done and you try and check and try and check until the holes are corrected.

 

[M1A's r BEST]

What am I hurting on my rifle???

I ask because what I've been doing (just once after sighting in at 25 yds) is counting the clicks to full bottom, loosening the screw in the elevation knob, setting the elevation knob so it lines up (yards wise for 250 for my BSZ) and then snugging up the screw while holding the knob and then using the screw driver to tighten up the screw after the knob turns around to the full up position.

Is this damaging the teeth on the sight or the pinion? Thanks.

 

[Gus Fisher]

In a shameless plug for my own product, I sell rebuild kits to replace hoods on a MN sight. If that little C clip comes unwound (usually during a match),

Art,

Got a chuckle out of that statement about a shameless plug. $15.00 for the whole kit is cheap insurance if you shoot any kind of matches with a hooded rear sight. I would recommend anyone who competes with them get one of these and put it in your shooting kit.

Why do I say that? Well, we could have used one at Camp Perry just last year. The comedy of errors began when a good buddy and Distinguished shooter friend of Mike Gingher and mine decided at the last minute to put NM sights on his M1A to compete. (Don't even get me started on the lack of wisdom about waiting till just before a match to change sights. Grin.) Anyway, I had one set each of fitted .595 and .520 NM apertures and RS bases. Mike and I suggested the .595 sights due to the our buddy's age. Well, they are out zeroing the rifle the morning before the match and guess what? Our buddy bumped the hood on something and it popped off. Now Mike and I have replaced many hoods on RS hooded apertures,but he didn't have a tool with him. I'm back in the Armalite Booth with the tools, but they really didn't have time to come back, get it fixed and go back out. So they installed the .520 aperture and hood. Well, it was a bit too small for our buddy's eyes and it would have been better had he or Mike had one of these kits to repair the .595 hood and get a good zero with it.

Oh, and Art is correct that sure as shootin when something goes wrong with a NM hooded aperture, it is almost always in a match or when you really need it. Murphy ensures that.

 

[Gus Fisher]

What am I hurting on my rifle???

I ask because what I've been doing (just once after sighting in at 25 yds) is counting the clicks to full bottom, loosening the screw in the elevation knob, setting the elevation knob so it lines up (yards wise for 250 for my BSZ) and then snugging up the screw while holding the knob and then using the screw driver to tighten up the screw after the knob turns around to the full up position.

Is this damaging the teeth on the sight or the pinion? Thanks.

No, you should not be damaging the pinion as long as you hold the drum of the elevation pinion with something that won't mar or crack it - while you loosen or tighten the screw. If your fingers are strong enough to hold the drum, then you are in tall cotton. Most folks' fingers aren't strong enough to hold the drum securely enough to do that, though.

BTW for the folks who don't know, this is basically how you use the engraved and numbered hash marks on the elevation pinion drum to get a basic zero on a rifle - as the pinion was designed. Then if you had to shoot at longer ranges, you were supposed to move the elevation pinion to the higher engraved and numbered hash marks. In reality and in the Marine Corps, we didn't use the hash marks on the elevation drum. We kept track of how many clicks of elevation were used for each yard line. In combat, we set the sights at a BZO of 300 yards and just held high for longer ranges.

 

[hueygunner]

pliers

gus:
those pliers are an esstential item for most aircraft mechanics.
they are for cannon plugs found on airframes engines and avionics and instruments.
technical name as per aviation types as well as dod types are pliers soft jaw cannon plug.
you can get them cheaper on ebay or right from any aircraft tool supplier.
gus you are a most knowledgeable guy and your posts are spot on. regards jeff shapiro hueygunner

 

[m1a shooter]

Thanks Gus, who needs tech manuals when we have the experts here at our fingers.

 

[ShootingSight]

Gus,

Send me your address,and I'll send you out a repair kit to keep in your shootig bag. Having the kit is a great way to guarantee that no sight will ever come apart again.

Art

 

[ShootingSight]

Oops, just noticed Hawk pulled my 'shameless plug' post, based of not allowing selling/advertising in this area.

Sorry Hawk, the shameless plug comment was meant in humor. I thought an indication that reapir kits exist was germaine to the discussion at hand.

Art

 

[Gus Fisher]

gus:
those pliers are an esstential item for most aircraft mechanics.
they are for cannon plugs found on airframes engines and avionics and instruments.
technical name as per aviation types as well as dod types are pliers soft jaw cannon plug.
you can get them cheaper on ebay or right from any aircraft tool supplier.
gus you are a most knowledgeable guy and your posts are spot on. regards jeff shapiro hueygunner

Jeff,

That's great info and should be of good use to many folks. Thank you.

 

[Gus Fisher]

Oops, just noticed Hawk pulled my 'shameless plug' post, based of not allowing selling/advertising in this area.

Sorry Hawk, the shameless plug comment was meant in humor. I thought an indication that reapir kits exist was germaine to the discussion at hand.

Art

Art,

I'm glad you did mention your repair kit and parts as most people don't know where to find such parts to fix NM apertures.

 

[Gus Fisher]

Part IV

The next thing to go over is the RS Base (hereafter referred to as the Base for brevity.) On many of both the standard and NM bases, the very front bottom edge of the base sometimes has a burr or slight downsweep on them from the manufacturing process. On some of the NM bases, the pad on the front bottom edge of the base was made a little too far down. On some NM bases, the bottom rear edge also sticks down a bit low. So I’ve come to take my ¾” stone and slide it sideways on the entire bottom of most bases I work on to get rid of burrs and see if there is such a high spot on the front bottom pad. A few strokes sideways, like one is draw filing, is all you need to do at this time to ensure there are no burrs. NOTE: The ENTIRE bottom of surface of the base is NOT entirely flat, though, as the rear of the base sticks down farther than the front. The main purpose of the Base sticking down in back is to ensure the rear sight cover won’t be popped out and as a dust shield – as far as anything I’ve ever been able to determine.. The base is meant to slide and be supported on the large machined flat portion of the receiver. Then you take the elevation pinion and windage knob off the receiver to go on to the next step.

Now you want to mount JUST the elevation pinion, RS base and windage knob. We are doing this to isolate any problems the RS base might have and fix them now, as much as possible. Tighten the nut on the windage knob to the “sweet spot” mentioned before and then turn the windage knob to center the RS base on the center engraved hash mark line on the receiver scale. Now it’s time to turn the windage knob so the Base goes ALL the way to the left. What we are looking for is if the scribed lines give us a good reading and if there are any bumps, catches or stops that slow down or stop the Base from moving to the left. The longer scribed lines on the receiver scale are supposed to represent 4 clicks of windage on a standard RS and 8 clicks on a ½ minute NM RS Base. What we are really looking for is to see if there is enough clearance for the Base to move freely.

If the holes for the windage knob and elevation pinion were drilled down too far close to the surface of metal the Base rides on, that will cause hard turning of the windage knob or it will freeze up the Base when using windage adjustments. If that is going on with a Standard G.I. type Base, it is time to stop there. You either need a skilled armorer to look at it to see if it can be freed up or if it has to be sent back to the factory. Send it back to the factory if you don’t have someone experienced with working on Garands or M14’s to assist you.

If you feel some extra bumps, catches, etc. – what may also be happening on a commercial receiver is the flat shelf behind the rear sight ears of the receiver may not have been machined correctly. I’ve seen some pretty large burrs or insufficient machining back there. You can flatten that out with your ¾” stone and smooth it up as required. That shelf is basically just a clearance cut as the base rides on the machined surface above and in front of that shelf. So we are not concerned if you go a bit too deep clearing that shelf. BTW, if the holes for the pinion and windage knob were drilled too low on the receiver, the rear bottom of the Base will also bind up there.

If the machined surface for the Base is higher towards the left side of the machined cut on the receiver, that will cause the Base to begin to bind as you move the base towards the left. I’ve had to use the ¾” stone to take one side or the other down a bit so the Base won’t bind as it gets close to one side of the receiver. I’ve even had to use a diamond file on a very few receivers. If that is causing the binding, then it is time to get an experienced Armorer to look at it or send it back to the factory.

The next thing to check is to be real careful to look at the Base when it gets close to the left receiver ear. You don’t want the Base to be rubbing on the top of the receiver ear. This was a real problem in the early 80’s on SAinc. receivers and some receivers of other manufacturers since then. I still run into this occasionally with commercial receivers. What you do is use a rotary stone or sanding cartridge roll to clear the top of the receiver so the Base will ride over it without touching. Black magic marker or Dykem blue on the receiver ear will show you if the Base is rubbing on it. Once it is clear, use sand paper for metal to smooth it up. NOTE: I’ve been asked about clearing the Base instead of the receiver as the Base is the cheaper part. In this case we want to work on the receiver so if you or someone else ever decides to put another Base or NM Base on it, it will fit correctly. It does not matter if you cut through the hardness on top of the receiver ears as there is no stress on them.

Once the Base moves freely all the way to the left, do the same things and move it all the way to the right. Also do any clearing or stoning as required.

It is very, VERY unusual that the Base will bind or hang up on the very front bottom of the Base on that pad when you move it for windage at this point. That’s because there is no tension on the front part of the Base at this time. However, if the front bottom of the Base IS binding or hanging up and a few more strokes of the stone won’t clear it, then it is time to seek an experienced Armorer or send it back to the factory.

If the base has clearance and somewhat moves freely, but you notice something hinky or just not right about the way the base moves, I also suggest you try a different windage knob. What is possible, but is rather rare, is the C spring that gives tension to the nut in the windage knob may be wearing out. Trying a different windage knob should tell you if that is going on or not. This can also be caused by the spring in the pinion beginning to wear out.

At this point, you should be able to move the RS base all the way to the left and right side of the receiver without binding and so the engraved hash marks on the receiver scale align with the engraved line on the back of the Base. The scale on commercial receivers may be off a bit and we really can’t fix that. You will be keeping track of how many clicks of windage you use at every yard line anyway, won’t you? So that really isn’t a problem if the scale is not perfect.

We MAY have to work on the bottom of the RS base some more as we go along, so this is a case where we have to check and perhaps do more fitting on down the line.

Going to end this post and the next post or two will be on the Aperture.

 

[ShootingSight]

Gus,

I love the detailed explanations, and was absolutely tickled by your observation that the aperture hole has to be round, or it will draw off your focus.

Without going into the optical math, I will say that I agree and disagree. Yes, if it is non-round, it will create a bias in your focus.
However, round is not the best shape.

I invented a rectangular aperture for the M-1A based exactly on the observation that for an opening of a certain size (ie constant brightness), if you ovalize the hole, it will bias your focus. If you make it wide horizontally and small vertically, it will improve focus on horizontal lines more, and diminish focus on vertical lines, and as long as you keep the total open area of the aperture constant, the brightness will not change.

As it turns out, for a front sight post, a round aperture is not the optimum shape, since your needs for horizontal focus and vertical focus are different. To judge elevation, you need great focus in order to put the top horizontal edge of the front post up against the bull. To judge windage, you are balancing the bull sideways between two vertical edges of the post. So if I can improve your focus on horizontal edges slightly by giving up some focus on vertical edges, this is a good trade.

Your brain has agreat ability to center things, so as long as your front sight has two edges, your eye can center the bull side-side between them, it does not matter if the sides of the front post are sharp, or fuzzy. But for elevation, you don't have the ability to center, because you only have one top edge to the post - so you need sharp focus to judge precise alignment.

I know this is true, because when you get an older shooter whose eyes have gone, the typical symptom is that they string their shot groups out top to bottom. This is because even with blurry vision, they can still center the bull to judge windage, but with blurry vision they cannot judge elevation.

I'm not the first person to figure this out - Mother Nature beat me to it. Ground level hunters, who need to focus btween vertical edges to find prey, because they are surrounded by grass (which is mostly vertical edges), have evolved slit shaped pupils. Cats, snakes, and some others have evolved a non-round pupil to help them preferrentially focus in one direction.

Thus, by replacing the round aperture with a horizontally oriented rectangle, I improve focus for elevation, by giving up slightly on focus of vertical lines. WIth a front post, this trade represents a net gain by improving a critical parameter in exchange for giving up a less important one.

Instead of a .052 round, I offer a 0.030" tall x 0.075" wide (same area - same brightness), but instead of the focus of a 052 on your horizontal edge, you get the focus of a 030 aperture on that horizontal edge.

I also make a 036 x 080, which lets in the same amount of light as a 0595 round.

Testing using ARs suggested there was an average 15% reduction in group size by using a rectangular aperture.

Art

 

[Gus Fisher]

Gus,

I love the detailed explanations, and was absolutely tickled by your observation that the aperture hole has to be round, or it will draw off your focus.

Without going into the optical math, I will say that I agree and disagree. Yes, if it is non-round, it will create a bias in your focus.
However, round is not the best shape.Art

Art,

You have done some remarkable brainstorming on using rectangular apertures. This is yet another example of how we all can learn something new. Is there a size rectangular aperture that you recommend as "better or best" for someone over 40, over 50, etc.? Also, do you know how astigmatism affects the use of the rectangular aperture? This has always been my personal problem and is the reason I was never able to be a Match Shooter.

But back to the general subject at hand on standard G.I. type apertures..... What I'm trying to get across to people is that if the standard G.I. aperture is dinged up or out of round, it will cause them problems in shooting as accurately as they would with a nice round, clean hole in the aperture. So if they want to keep to the original G.I. design of the rear sight, they should replace one that does not have a dinged up hole. They could also use a hooded rectangular hooded aperture, to be sure, but let's get rid of the apertures that have dinged up holes. Grin.

 

[ShootingSight]

Gus,

Your questions are right on it.

1. There are limits to how rectangular you can go. In width, you never want the aperture to be wider than your pupil, otherwise it is you pupil that sets the depth of field, not the aperture, and since the pupil varies with brightness, your focus will be all over the place. Furthermore, if you are close to the same size, you get the risk of parallax error. If your head is slightly out of line, the image that enters your eye is bounded on one side by the pupil, and on the other by the aperture, which will cause things to appear to move sideways. Bottom line, around 0.080 to 0.100 is the maximum width, Pupils can get as small as 0.125", but that depends on lighting and the person in question.

2. For rectangular height, at some point you start seeing diffraction errors, als known as dust bunnies, or spider webs. Most people can see through a 0.025" slit without major issues, especially if the light is good. I have a few folks using 0.023, but that's minimum. Grab a pair of calipers, and hold them in front of your eye at the same distance as your rear sight (this is important) and open/close them slightly to see where you start seeing the image go dim, where you start seeing parallel lines. If I hold them close, I can get an image down to 0.020, but at rear sight distance it's more. Hold your finger out at arm's lenght, and use the calipers. If you hold the gap horizontal, I'll bet you see blur lines on the sides of your finger, and not on the top. Or just look at a roof line - if the calipers have the slot horizontal, all vertical lines you see will have much bigger blur than the horizontal lines you are looking at.

3. For astigmatism, these things work great! Instead of putting them horizontal, you align them to cross the axis of astigmatism in your eye. Basically, this is an adaptation of the slit that eye doctors used to use to diagnose astigmatism in people (at least before they invented laser scanners that just map the topography of your cornea). If you have astigmatism, your eye lens is shaped liek a football, instead of a basketball. As a result, objects that line up with different diameters of your lens all get slightly different focal treatment. A line from 4 oclock to 10 might be in great focus, but a line from 1 to 7 is miserable.

If I am going to design a slit shaped sight, that only lets a slice of image fall on your eye, why don't I align that slit so it falls on exactly the best slice of your eye?

Trust me, I can add 15 points to your score at 600, more in XTC, and put you in the game.

Art