What is tuning an airgun? Why Tune an airgun?
Usually we mean to make it more efficient and more Accurate and powerful. Like mentioned earlier, most guns are not very close to the legal UK limit of 12 ft lbs. The different types of airguns will require different methods of tuning. A lot of amateur tuners think that you just put a large oversize and far stronger mainspring into a spring piston airgun, you get more power. While you can use a stronger spring it is best to modify the inside to get the air out faster with less drag. If the air cannot get out fast enough then you will get more recoil and less power than a better balanced spring that matches the air flow.
Remember that it is no good having a lot of power if the gun is not accurate. Accuracy is the first thing required. Secondly you need enough power at the point of impact to kill vermin humanely.
The tuning of the different types of airguns varies according to the way they work.
We do the Airgun Tuning Manual for Spring Airguns. This covers the principles of tuning spring guns.
Pump up (pneumatic)
Air guns are tuned by making the pumping and firing more efficient, the pump rod can be made adjustable and the reservoir made to give the best performance. This is harder to do for the UK because you have to set it so the gun cannot exceed the legal limit, even if it is over pumped.
For FAC we normally recommend buying the gun from the factory set at 25 ft lb or below. If set higher than 25 ft lb, the accuracy will suffer and the number of shots will be a lot less. The power curve of an unregulated gun will also be more curved.
Spring guns are tuned by reducing friction and making the most efficient use of the spring power available.
If the spring gun is not tuned correctly then the power and range will actually be reduced, which is the opposite of what you are trying to do.
Below is picture of a gun tuned by an amateur. This spring gun was hard to cock and only produce 8 ft lbs. The poor spring had broken the spring guide and hammered the end.
Tuned properly the gun produce 11.5 ft lbs. and was not that hard to cock. We used one of our 29 seven springs made from Swedish Silicon Chrome, and our Steel spring guide etc. The calibre was .177.
When Tuning a PreCharge it is important to tune, using efficiency rather than brute force. Brute force is used by amateur tuners and is what I see the most with so called tuned PreCharge. Using brute force to get more power will make the gun much less reliable. If your gun is hard to cock then check that it has not been tuned by an amateur. You are probably better off having a go yourself than letting some one else try his hand on your gun.
The valve length will affect the opening time of the valve, but it will also be affected by the hammer spring and the pressure inside the reservoir. The valve length is measured from the end of the valve to the valve face. See picture below. If the length is too long then the power will be low, and if it is too long then the power will be low. The best setting is somewhere in between. Use the old valve as a guide to get the optimum.
Carbines, a Carbine generally means a rifle with a short barrel. The shorter the barrel length, then the more air you will use, to get the same power of a longer barrel. Also with a PreCharge it will usually mean a shorter reservoir, that is why carbines will have about half the number of shots than that of the rifle version.
Accuracy is dependant on the quality of the barrel more than the length.
Spring powered guns are not so much affected but the barrel length will affect the power.
A very long barrel will also reduce the power, but this would not normally affect the gun unless it has no air pressure left in the barrel at the time the pellet leaves the muzzle.
A good indication with PreCharge, Pump Up or Co2 guns is the report (without the silencer). If the gun makes a loud Bang then that is wasted Power.
Getting more Air
One way to tune a Spring piston airgun, is to get more air inside. This can be done by increasing the stroke and or the bore. The more air you can compress, then the more power can be obtained, up to a limit of airflow velocity. You will also need a spring powerful enough to create enough energy. Any lost volume will reduce power. Long transfer ports or large ports will reduce power.
To be continued.
TR Robb Airgun Specialist
316 High Road,
South Benfleet, Essex. SS7 5HB 01268 752888
BSA Airsporter / Mercury Piston Modification to increase the stroke.
This does away with the buffer washer and increases the stroke by the thickness of the buffer washer, approximately 5mm. This modification is for the later BSAs Not the early Mk 1 & 2.