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I am Often asked all sorts of questions about all types of Air Rifle designs but one of the most popular inquiries is in relation to the swept volume and piston diameter used for spring rifles. There is a popular misconception that it is the most important factor in producing the relevant power of the rifle and I suspect in some way it goes back over the last one hundred years of producing rifles of which the only visible difference between two rifles of differing power levels was the length of the power tube and corresponding piston travel. This was especially true of the early Lincoln Jefferey and BSA rifles. However, it is important to remember that back in those days most rifles relied upon a percentage of dieseling to give the power levels that they achieved. This did give more power from the swept volume they used but it tended to mean that the true power varied quite a lot from shot to shot as it was dependent upon the amount of oil being burnt each time. Nowadays when power levels are measured we use clean air for each shot which does not produce as much power per cc but is far more consistent.
Having discussed this topic with many people over the years it has become obvious that whilst there are several companies still producing spring powered Air Rifles, there are very few Designers who fully understand them sufficiently to be able to produce a totally new and efficient one.
I have had a real interest in spring powered Air Rifles ever since I got to use my first one (an Original Mod 25) back in 1957 under the close supervision of my Father. Over the years I have done many thousands of hours development on them because the only way to learn everything is to keep pushing the envelope until it does not work and then to discover why it didn't and fix it. In this way we gain another asset in the Toolbox of knowledge which in turn helps us to produce better and more efficient products for the future.
* It is interesting to note that the main performance difference between the way, a spring rifle propels its pellet compared to a PCP : is that the PCP uses cold air and the spring rifle uses hot air, heated up during the firing cycle. A PCP requires 0.6ccs of air at 200 bar to be released instantaneously behind a 5.5 mm (.22") pellet in a good fitting rifled barrel that is 400 mm (16") long to achieve 12 ft/lbs of energy at the muzzle, that is 120ccs of free air (ignoring losses), whereas a well-designed spring rifle will easily do it with 35ccs of swept volume. However, if the calibre of the rifle is changed to 4.5 mm (.177") then an increase of around 15 to 20% of energy (depending on rifle design) will produce 12 ft/lbs of energy from the same 35ccs of swept volume with the spring rifle but the PCP requires 50% more air IE 180ccs to achieve the same level of power. Conversely, if a .22 barrel is fitted to an existing .177 12 ft/lb action then it will usually produce around 17 ft/lbs of energy which is why in the UK we do not tend to have dual barrels for our Air Rifles. Please note that the knock open valves, used by most manufacturers expel considerably more air than this, unless of course they are using GET designed ones. (sorry about that bit of advertising folk's)
With regard to the swept volume: In the UK where a rifle with its power level as near as possible to 12 ft/lbs is the goal 35 cubic centimetres of air appears to be the magic figure. However, whilst I can produce considerably more power from this volume of air, it is very difficult to produce 12 ft/lbs from very much less and there are a great number of factors that effect it. These factors include: (1) Transfer port size, shape and position. (2) Piston weight, head and seal design. (3) Spring type, stiffness and design parameters. (4) Lubrication. (5) Breach type and pellet fit. (6) Barrel size and rifling type. (7) A crisp and precise trigger release. (8) Type of weapon IE Break barrel, Under leaver, Side leaver or some other configuration.(9) Lock time: shorter lock times usually generate hotter air.
With regard to piston diameter: there are many schools of thought on this but the most important factor is the magnitude and timing of the pressure pulse going through the transfer port that drives the pellet out and ideally it will do it without too much recoil. It is difficult to produce high enough pressures with a large piston unless excessive amounts of weight are used and excessively long springs are required if the piston diameter is too small. Those of us that have done sufficient testing now have a fairly good idea of what piston diameter to stroke ratios work best for the degree of power we are trying to achieve and to satisfy all of the other parameters involved.
With regard to springs: one of the most important factors usually ignored by Air Weapon designers is that it should be pre loaded to 15 to 25% of its stroke and then only be compressed to the 75% point if it is to be left in the cocked position for any period. However, a lot of modern Air rifles have springs that are compressed to over 90% of the available stroke which is why they usually do not last very long. There are a few springs now being made that will work under higher stress, but they tend to be expensive and are prone to breakage. Air Rifles that I have tuned and re-sprung using the 25 75% rule rarely ever need a new spring again after that. It does seem wrong to keep a spring at full tension when the piston has to move half of its stroke to increase the pressure in front of it to 2 bar. Part of the reason that I have been getting involved with gas spring design is that it is now possible to make them with a more suitable power curve to suit the requirements of the spring rifle.
With regard to recoil: most break barrelled spring rifles have a tendency to jump forward when fired as a result of all the energy being released by the spring as it drives the piston towards the transfer port wall at the end of its travel. Various ways have been tried to eliminate this effect such as a cushion on the piston head or having the action mounted on a slide and wedge system in the stock so that its movement is arrested in a gentler manner. However non of these are a perfect answer as they only work to remove the result of the effect. The best theoretical system is to have two pistons moving in opposite directions and meeting at the transfer port, so that any excessive energy of each one is cancelled out by the other as they stop. This Twin Piston type of design has been done quite successfully before but both rifles that I know of, have been quite heavy and I think this is partly due to the big swept volumes that they appear to need to produce the required power levels.
I have always thought that if you had two pistons sweeping the same volume in half of the time IE a Twin Opposing Piston Rifle, it should produce more power not less from 35 ccs of air. However, as it turned out this was a far bigger problem than I could ever have imagined.