Jim Tyler explains how ‘extreme tuning’ can potentially damage your rifle - and dispels some of the myths surrounding it
I’m no great fan of the use of analogies to explain anything to do with airguns, but there is an exception, and it is in one important respect comparing the spring airgun to the internal combustion engine. A production spring airgun is very like a production car engine, in that the airgun is designed to give many tens of thousands of shot cycles with only occasional servicing, and the production engine should be good for 100,000 miles and upwards, again with infrequent servicing. In contrast, a highly tuned airgun is like a highly tuned ‘race’ engine, which has to receive major attention at frequent intervals, up to and including a full rebuild after every single race; take things too far when tuning springers and they will be the same, but what is ‘taking things too far’ in airgun tuning? Well, it’s practically anything and everything that tuners do and have done in the past, from cramming in monster mainsprings in the early days of tuning, to the current fad of starving the gun of piston or air mass, but in both cases, taken to extremes.
Prior to the early 1980s, a surprising number of airguns produced in the region of 8 to 10 ft. lbs., but because very few of those who worked on airguns had the means to measure muzzle energy, they did not know what muzzle energy they’d started with, nor what they ended with, so many blindly fitted a much stiffer spring, and if the resultant recoil cycle was severe enough, judged the ‘power’ to have been increased. In fact, far from increasing muzzle energy, the consequences of over-springing were often to lower the muzzle energy, whilst making the guns less accurate, less reliable, prone to mainspring and other mechanical failure, and hopelessly inaccurate in many cases.
So, what was happening inside the over-sprung airguns of old? The spring was expanding extremely rapidly, increasing the severity of the recoil, generating higher cylinder pressure faster than the air could escape through the transfer port. After the piston and spring ran out of energy, this resulted in severe piston bounce and hence surge, followed by a very harsh piston landing at the end of the second forward stroke. Many of the airguns of the time had a design characteristic that limited their muzzle energy, and that was the limited available length for the piston stroke which, coupled with the high start pressure pellets of the time, gave a very short and brief cylinder pulse – the length of piston travel and time that the piston and pellet are travelling in the same direction, when the pellet needs to gain most of its velocity.
The only sure method of raising muzzle energy in many airguns at the time, without major surgery, i.e. lengthening the piston stroke where possible, was to ensure that autoignition (dieseling) gave a pressure and temperature spike to get the pellet moving earlier, to increase the cylinder pulse and give the pellet time to accelerate to a decent velocity before the piston bounced.
Because most airguns were fitted with leather piston seals that could act as a wick for the oil that fuelled dieseling, it was common practice to use a light mineral oil, with a low autoignition temperature, for cylinder
“… This is not to be seen as a ‘cure’ so much as a sticking plaster for a self-inflicted wound …”
lubrication. Because the autoignition necessary to get the muzzle energy of short stroke airguns to the desired level occurred at relatively low temperature and pressure, over-springing the air rifle was totally unnecessary. It served only to make the rifle more difficult to shoot accurately, as well as raise peak cylinder temperature and increase the chance of spring fatigue or failure, but because it increased recoil and surge, people continued to cram in far too much spring in the mistaken belief that the ferocious recoil was a sure indicator of ‘power’.
Affordable, reliable and accurate chronoscopes became available in the early 1980s, revealing that airguns fitted with overly long and stiff springs usually did not achieve the desired muzzle energy, and they fell out of favour with tuners, though most aftermarket springs were still stiffer and longer than was good for the guns in which they were fitted, a situation that sadly continues to this day.
Most people know that recoil is a reaction to the mass of the piston and spring powering forward during the compression stroke, and that reducing the piston’s weight also reduces recoil displacement. If the weight reduction is modest, or if you’re prepared to sacrifice a little muzzle energy, then no harm done, but go over the top at your peril.
Reducing the piston mass reduces the piston’s momentum, which is a big deal because it is the momentum that compresses the air. So, if you want to maintain muzzle energy, you need to increase the piston’s velocity by fitting a stiffer spring to get the momentum back up. Unfortunately, the lighter piston has lower inertia – resistance to moving – and at the end of the compression stroke, the high pressure air in the cylinder accelerates it back up the cylinder faster and further, lowering muzzle energy and increasing surge. You can compensate for this by either increasing spring preload to resist the force of
compressed air, for which you’ll probably need a softer, longer spring, or using a stiffer spring to drive the piston nearer the cylinder end, increasing peak cylinder pressure, and temperature. As the fashion today is for less preload, most go with the stiffer spring option.
The issue with high peak cylinder pressure and temperature will become apparent as you read through the article.
Friction, mainly between the piston seal and cylinder wall, was for many years seen as the greatest obstacle to increasing airgun muzzle energy, which has led many to experiment with a succession of super lubricants, sometimes with unexpected and undesirable consequences.
The most common ‘super’ lubricants are polytetrafluoroethylene (PTFE) and molybdenum disulphide, usually referred to as ‘moly’; in either case, in the form of tiny particles in a viscous liquid carrier, and it is the carrier that causes the problems, rather than the lubricant. Some carriers give off fumes that ignite during the compression stroke, just like airguns of old dieseling with light mineral oil, and there’s a potential problem if the amount of fumes given off varies with temperature. If the fumes from one shot reached the saturation level necessary for autoignition, but failed to achieve that with the following shot, there will be two very different pellet starting points, and two widely spaced pellet points of impact.
REPLACE TO REDUCE
The biggest reduction in friction comes from replacing a synthetic ‘parachute’ piston seal with an ‘O’ ring. During the compression stroke, as the air pressure rises, the synthetic piston seal is squashed, and presses ever harder against the cylinder wall, causing a huge increase in friction in the hundreds of Newtons – the friction rises exponentially, along with cylinder pressure. This does not happen with an ‘O’ ring, so the compression stroke is much faster, but – and it’s a big but – the friction between the synthetic seal and cylinder wall that slows piston bounce is not there, so the piston bounces faster and further, starting when the pellet would still be accelerating with a parachute seal. To compensate, peak cylinder pressure must be raised, which means peak cylinder temperature is higher.
REDUCING AIR MASS
The easiest way to reduce air mass is to shorten the piston stroke, which is currently very much in vogue, and which reduces recoil, as well as quickening the shot cycle to pellet exit. Because most full-sized spring airguns are destined not only for the UK market, but also markets where muzzle energy is unrestricted, most are designed with longer piston strokes than strictly necessary for our 12 ft.lbs., and a modest reduction in stroke can be undertaken without any undesirable side effects. However, some tuners like to take things to extremes, and go OTT.
Reducing piston stroke to excess is effectively turning the clock back to the bad old days of the late 1970s, when most air rifles had short strokes, and we all struggled to get them to produce muzzle energy as already described.
There is another way to reduce air mass, which does not involve shortening the stroke, and it is to fit a sleeve in the cylinder to reduce the diameter. The attraction of cylindersleeving is that it allows a less heavy piston to be used, reducing recoil, without the drawbacks already detailed, and the reason why is that it maintains the piston’s sectional density and resistance to bounce, whilst lowering the force from the compressed air that causes bounce, which is proportional to the piston’s cross-sectional area..
If you sleeved a 25mm cylinder down to 23mm the force driving bounce would reduce according to the square of the diameter, and the piston mass could be reduced by the same amount whilst maintaining sectional density, so a 250g piston could be reduced to 211g, making a big reduction in recoil. Sounds good, and doubtless is up to a point, but if you go over the top, there’s the potential for nasty things to happen.
I have been following Internet forum posts from people using sleeved cylinder and short stroke airguns, and while many report no problems, some describe multiple mainspring breakages, others fried breech seals.
The reported breech seal damage is caused primarily by extremely high temperature, probably exacerbated by accompanying extremely high air pressure, and both are a result of the tuner going over the top to compensate for problems of their own making, which can include over-shortening the available piston stroke, overdoing a reduction in piston mass, or going too far when reducing the swept volume. There seems to be a relationship between available air mass and the maximum muzzle energy that can be achieved before peak temperature becomes high enough to damage seals. I have witnessed breech seal failure following the very first shot of an airgun that was pushed beyond the ‘natural’ muzzle energy level for its swept volume, and I have seen cumulative breech seal failure with an airgun that had a combination of too light a piston and too little air mass. In both cases, manifest as a sudden and large fall in muzzle energy, and in both cases, the seals looked as though someone had played a blow torch on them.
REDUCED AND DELAYED
Temperature related breech seal failure can be reduced in some cases, or perhaps delayed, by chamfering the transfer port outlet, which provides a small volume into which the extremely high temperature air can expand before it reaches the seal and, on expanding, the air cools. This is not to be seen as a ‘cure’ so much as a sticking plaster for a self-inflicted wound; the only cure is to reverse the causative modification to an extent. Give a little mass back to the piston, give the gun more air mass by increasing the piston stroke, and take away some of the spring energy.
By no means all, but the majority of mainspring fractures seem to occur about an inch from the rear of the spring, and one plausible explanation is that the spring in this
“… the seals looked as though someone had played a blow-torch on them …”
region is still expanding when the coils in front of it are compressing after piston bounce, and the fracture occurs where the two wave fronts meet, probably not on shot one, but following repeated wave front collisions. Although the wave front collisions are a function of timing, there can be little doubt that the chances of fracture are increased the higher the stresses on the spring.
If the reason for the sleeved cylinder rifles breaking mainsprings is indeed colliding wave displacement fronts, then the culprit will almost certainly be ferocious piston bounce due to very high cylinder pressure at, and following, piston bounce. Again, the solution is to add back a little swept volume, add back a little piston stroke or piston mass, and take some of the strain off the mainspring.
There is another potential cause of high spring failure rate, and it is coil diameter. If the diameter of a piston is reduced to, say 22mm, then the spring diameter must also reduce to fit inside the narrower piston, to the point at which the spring is significantly exceeding its elastic limit in use, leading to creep (shortening of the spring without weakening it), fatigue (shortening and weakening the spring), or breakage.
Perhaps to address the problems with small diameter springs, perhaps for ease of manufacture, some tuners have experimented with ‘skirtless’ pistons, which can only work in airguns that use a sliding cylinder to compress the mainspring. Skirtless pistons are nothing new, but in the past, they have tended to feature in very cheaply made, low muzzle energy airguns, with springs that were not over-taxed, and my concern with skirtless pistons in airguns producing near 12 ft.lbs. is that if the front end of the spring is poorly supported, it might go out of line and adopt the familiar ‘S’ shape more quickly. Time will tell whether my concern is founded.
Perhaps when it comes to spring airgun modification, the old adage, ‘Everything in moderation’ might be worth bearing in mind.I
The Original (Diana) 45 was the first true magnum airgun designed for unrestricted markets, and the first I short-stroked for UK levels.
Prior to the 1980s, the Airsporter mainspring was widely believed to raise the muzzle energy of any spring airgun. It did not.
Go too high with peak cylinder pressure and temperature, and you can scorch breech seals. Spring breakage is commonly caused by inappropriate heating and cooling during shortening, although ferocious piston bounce can also be responsible.
Extreme cylinder temperature and dieseling do piston seals no good at all.
Taking internal changes too far can make rifles much harder to shoot well.
Manufacturers as well as tuners occasionally go OTT, such as the 22” barrel of the HW35 Export.
If there’s insufficient swept volume, no spring is going to achieve a desired muzzle energy.
The fashion is for stiff springs with minimal preload, which certainly makes assembling airguns easier, and safer.
Square section springs used to be commonplace, usually crudely ground down to be crammed into an unfortunate airgun. Bad idea on every level.