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Author Topic: Air rifle sights.  (Read 1572 times)

Offline Novagun

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Air rifle sights.
« on: June 17, 2013, 07:52:16 PM »
There are five types of sights that can be used on an air rifle. They are an open sight, sometimes called a V sight or iron sight an aperture sight sometimes called a peep sight: a telescopic sight, a laser sight and a holographic type known as a red dot.

The well known open sight has post or blade at the front of the barrel and a bar with a V or U shape cut in it. The idea is to line up the post with the top V and the target appearing just above the lot but in the centre. Squeeze the trigger and a neat hole appears in the centre of the target, exactly where you were pointing the rifle. Just dead easy. The bar with the V cut in it is adjustable for windage, that is side to side and adjustable for elevation, that is up an down. Fine adjustment can be made using the screw to make changes in windage or elevation. If the pellet is hitting the target to the left of where you intend then screw the windage screw to the left; if it is hitting too high then adjust the elevation screw down. A few fine adjustments and the next shot-- bullseye. This type of sight is simple, easy to use and accurate. There goes the opinion part. For deadly accuracy the system is limited by the eyesight of the shooter and because the post and V are fairly course, can obscure part of the target making precise aiming a bit of a guess.

The next type; the aperture sight consists of a sight with hole in it at the muzzle. The rear sight consists of another device with a hole in it of about one millimetre diameter and close to the aiming eye. The rear sight has a micrometer adjustment screw for each of windage and elevation adjustment. To shoot just line up the target through the hole in the foresight and the hole in the rear sight exactly in the middle of both holes. Squeeze the trigger and another bullseye. This type  enables more precise aiming but is again limited by the shooters eyesight. On top of that the sight places strain on the eye and suddenly the eye can go blurry or imaginary lines or even cobwebs will appear. The only remedy is a short rest and start again. With practice this type of sight is very accurate.

The third type of sight is the telescopic sight. This type is a rifle accessory and adds to the cost of the rifle but is becoming very common and the most preferred. The advantages are that the magnification and light gathering properties overcome eye weakness. The disadvantages are cost, extra weight and fragility of the scope. They do break but not often.

For a spring powered air rifle with its characteristic recoil only a scope that is air rifle rated will survive. Any scope not so rated, however expensive and of whatever quality will be destroyed. That does not mean that air rifle scopes are wildly expensive. In fact some very good air rifle scopes are easily afforded.

A Telescopic sight, known as a scope comes in different sizes and are rated by the power of the magnification and the size of the objective lens. That is the one at the front end. The lens near the eye is called the ocular lens. So a 4 x 40 scope has a four power magnification, that means the target looks four times as close compared to the naked eye and the objective lens is 40 millimetres across and because of that has good light gathering capability. You can see the target clearly in normal light.  More expensive scopes can have variable magnification some 4 to 9 times, to 24 to 32 and even up to 50 magnifications. The more powerful the magnification the more light the objective lens needs to gather so it needs to be bigger. A very powerful scope will have an objective lens 50 or 60 millimetres across so that sufficient light enters the scope so that at high magnifications the target still appears bright.

All scopes have  an  adjustable ocular lens. That lens can be screwed in and out so that the cross hairs known as the sighting reticule appears sharp. To focus the ocular lens one should mount the scope on a rifle using the scope mounts that come with it or that are purchased separately.  The scope should sit on the rifle about 2 or three inches from the eye so that the whole lens shows the target picture without any thick black edge. The picture you see through the scope is called the field of view and you want it as large and clear as it can be.  Now point the the scope at a distant bland surface, a cloud is perfect. Look quickly at the cross hairs and look away. Screw the ocular lens in or out and look again. Keep repeating the procedure until the cross hairs appear clearly defined. Do not look at the cross hairs for long, keep doing it in short glances. The reason for this is that when gazing for a long period the eye naturally focuses on the cross hairs and when it does it is not in a relaxed state. Every time you look at the cross hairs your eye has to refocus to get a sharp picture. That takes time and effort.

Now is a good time to check that the screws holding the scope to the rifle and the scope in the mounts are tight. If they are not the scope will move especially with a high powered spring rifle. Make a couple of pencil marks on the scope and the rifle so that you can easly spot any movement. Be cautious about overtightening screws as they are steel screws working in light alloy and overtightening will damage the mounts or the body of the scope. If there is movement a strip of double sided sticky tape will usually solve the problem. Movement destroys accuracy.
Once you have properly focused the ocular lens you will not have to do it again until someone else uses your rifle and changes the focus to suit their own eye. Then you will have to do it over again.

It is important to mount the scope so that it is correctly aligned with the rifle. There are two methods to do this and usually both are used as a check. First mount the rifle stock in a vise. Take care and use padding. Place a small spirit level on a flat surface such as the breach block and adjust the rifle until it is dead level in the horizontal plane. Now put the spirit level on the top of the verticle adjustment turret and adjust the body of the scope until it to is dead level in the horizontal plane. Now tighten the screws on the scope mounting rings. Often the tightening of the screws will twist the scope out of level so keep fiddling until it is right. Now shoulder the rifle and aim at a string with a plumb bob on the end. The vertical cross hair and the string should be exactly in line. Keep fiddling until it is right. The way a salesman will mount the scope on your rifle for you by bolting it on, having a look and tightening it up is not good enough. A scope is a very precise sight and deserves a little close attention.

It is important to get the scope mounted correctly. If it is out of level, that is with the verticle cross hair exactly disecting the bore of the barrel at the muzzle, every time you adjust a cross hair instead of moving the point of aim straight up or straight across the adjustment is at an angle. So if when holding the rifle straight, as it should be, and not canted to one side the cross hair adjustment that should be taking the point of aim straight up or down is actually taking it across at the same time. That is not an easy way of accurately sighting in your rifle.   

It is best to get the rifle shooting with the cross hairs sighting accurately when they are at the centre of the lens. This is where the best picture occurs and it tends to deteriorate towards the edge. This is more so in modest priced scopes than expensive ones. Quality always costs money.

Now with the scope properly mounted you need to go through the process of sighting it in. To do this turn the top, that is the elevation screw all the way in one direction. Be gentle. Now turn it all the way in the other direction and count how many turns from side to side. Now turn the adjustment back half the number of turns. Repeat this for the windage screw on the side of the scope. That is called finding the optical centre of the scope. Now load the rifle and fire a shot at a large piece of paper with a clear dot in the centre. Use a range of about 10 to 15 metres or yards because scopes measurements are in yards. If all is perfect the shot will go through the dot. It won't, you can guarantee that so you need to keep shooting and adjusting first the elevation and then the windage by screwing the turrets knobs. A turret twiddlers delight, a point and shoot man's chore. The adjustment required should be slight.

You might find that the adjustment needed takes up most of the travel on the turret knob or there may not be enough available to get your shot on target. This will be because the scope mounts are not holding the scope in the right place. It is usually only in the verticle or elevation plane. To correct the misalignment use a shim under the scope in either the front or rear scope ring. If the scope is shooting too high then insert a shim in the rear ring. A strip cut off a coke can makes a good shim and a strip off a beer can is even better. The material is about .2 of a millimetre thick and one, two or three layers should be enough. Keep trying until you get it right but there is a limt before tightening can distort the scope tube. 

On occasions severe inaccuracies can be caused by a bent barrel. There are many of them. They are soft and it happens. That is not the end of the rifle. The barrel can be straightened by careful rebending. A good yank in the vice can do it or using a padded piece of pipe. Just do it a little bit at a time and keep testing your progress with test shots.

The more desirable scopes have an adjustable objective lens, that is the front one. That lens enables the shooter to focus the scope on a target at any distance. Apart from giving a sharp picture of what you are shooting at the adjustment focuses the target image in the same place as the sighting reticule. That means that point of aim on your target is exactly where the cross hairs on the reticule show it to be. You can then look through the ocular lens of the scope with your eye looking through anywhere on the lens and the cross hairs remain on the point of aim.  If that focus is not in the same plane as the cross hairs then the cross hairs will only give a true point of aim if the eye is looking through the exact centre of the lens. If the eye is looking through at any point other than the dead centre then the cross hairs will not show the exact point of aim. The error so introduced is called parallax error. It will greatly affect accurate shooting. You can test for parallax error by focusing on a target and then moving the eye slightly from side to side. If the cross hairs move on the target then parallax error is present

Scopes that do not have an adjustable objective lens have a fixed objective and for air rifle scopes is 10 or 15 yards and for powder burning rifles 100 yards. These measurements as still quoted in yards not metres. A focal distance of 100 yards is too long for nearly all air rifle shooting. There will only be zero parallax error in this type of scope when looking through the dead centre or went sighting at the fixed focal distance.   

A telescopic sight is the most accurate of all because it magnifies the target and allows the shooter to take the finest bead. They are so good that any shooter can still miss but that is not the scopes' fault.

Nearly all the photos of rifles in this book have telescopic sights mounted.

The red dot sight is an electronic device that displays a red dot on a lens and when correctly adjusted the dot superimposed over the target shows the point of aim. The sight is adjustable for windage and elevation and has changeable dot shapes.

Photo


A Lazer sight can be a unit like a small torch that is mounted beside a scope or it can be an integral part of the scope with the light shining out of the lens. The lazer shines a dot on the target. Like all light it disperses with distance so at long range the lazer dot will be diffused.

Photo.

Now there is another complication to consider. When you shoot your rifle the pellet comes out of the muzzle and as soon as it does gravity takes hold and pulls the pellet down. It is the same rate of drop for all rifle profectiles. Air rifle pellets drop more per unit of range than other rifles because the pellet is travelling slower. At long distances the air rifle should be aimed a bit higher to allow for the drop. This is called hold over. The drop associated with high powered rifles is the same but because the bullet is going so much faster the time per unit of range is less so the bullet drops less. All sights can be adjusted to take account of the drop a specific ranges and have to be readjusted for other distances.
   
 
« Last Edit: January 20, 2014, 07:10:20 PM by Novagun »