Blog / musings...

16^th Jan 2019 – Adding a regulator I'd been using a CO² powered air pistol (Crosman 2240) for sports competition use - but found early on that even small changes in ambient temperature would affect the speed of the pellet... making accurate aiming rather challenging, to say the least. As it happens CO² is one of the most efficient means of driving a pellet (far more so than compressed air). When CO² enters a barrel (behind a pellet) it changes from liquid form into a gas which causes it to rapidly expand and drive the pellet out of the barrel. The conversion process is however heavily dependent on the temperature... being most rapid when warm, but slowing considerably when cool. Pre-charged pneumatic (PCP) air pistols don't suffer from this problem because they use compressed air to drive the pellet instead of CO².

The SMK Model PP700W is a pre charged pneumatic (PCP) air pistol originally sold by SMK either in .22 or .177 calibre and which came with a characteristic and rather "loud" green grip. Newer models appeared a couple of years ago branded as Artemis with the model number PP700S-A. These had a black grip and a squared off barrel shroud with dovetail, but the organisation of the internals was the same. Both models are single shot, employing a novel rotating breech design and pull back hammer. They are inexpensive, accurate, have a good balance and can be used either in a cradle or arms-length style of shooting. They are ideal for competitive sports shooting on a budget, out to around 25 yards - and are great fun to use.

The PP700W is built with a high pressure reservoir capable of holding a maximum of 220 BAR (3191 PSI). It comes with a mechanical regulator designed to control the precise amount of low pressure air that will be released to drive the pellet whenever the trigger is pulled. The precision of this control is very important if the gun is to be accurate and once the regulator has reached its goal pressure, then any further pressure change should effectively cease. Well, that's the theory anyway... Any new air gun should be carefully checked in a controlled environment to assess pellet speed and in particular the consistency of speed over many hundreds of test shots. Fired in a safe location into a steel trap these shots are also passed through a chronoscope to measure pellet speed - the objective being to ensure that all shots fall within legal limits but also to quantify the variability across multiple shots. A huge number of factors affect accuracy when it comes to air guns (this is one reason why the sport is so interesting)... but a regulators ability to precisely control the speed at which a pellet is driven out of the barrel is a very big factor. When a target is 50 yards away, a variance of ten feet per second for a typical .177 pellet results in a movement to the point of impact (POI) of around 3mm whereas a difference of 50 feet per second results in 11mm movement (close to 1/2 an inch). Generally speaking the closer the speed of each shot can be to its neighbour, the better will be the result but allowing for the fact that regulators are mechanical devices, a variance of around 10 feet per second is perfectly usable. Initial tests with the PP700W out of the box, were disappointingly awful. Shot deviation was very large (around 75 FPS) and worse if the interval between shots was altered, the variability between shots increased. If the pistol was left overnight, a tell-tale symptom was that the first shot taken the following morning would be as much as 75 FPS lower than before. This particular symptom is highly characteristic of a leaky regulator... where high pressure air on one side of the regulator is slowly over time leaking across the air valve, building pressure in the low pressure side beyond the intended pressure. When that occurs, the pressure behind the firing valve (and which is holding it closed) increases and because the energy driving the hammer to open the valve during firing remains a constant, the final pellet speed will consequently decrease the next time a shot is measured. It is a very common problem with regulators and is sometimes referred to as regulator creep.

After a strip down and a significant number of tests, the deficiencies of the manufacturers regulator really became obvious. The design is straightforward - using a brass piston opposed by a conventional Belleville washer stack (left side of picture) and which opens or closes a valve made from a narrow tapered screw and a delrin washer (see the right side of picture). The piston is arranged so that it has atmospheric pressure on one side of the crown and the regulated output (low) pressure on the other side of the crown. With the pistol empty, the belleville washers relax, the piston rises in its bore and opens the connected air valve. After the reservoir is filled, high pressure air flows through the open valve to fill the low pressure chamber. Given the piston has atmospheric pressure on side, then as the pressure builds in the low pressure chamber, the piston experiences a pressure differential and so starts to compress, while being opposed by the domed belleville washers. When enough pressure differential is present, the opposition from the belleville washers will be overcome and the piston will start to move down in its bore flattening the belleville washers as it does and consequently closing the air valve. This movement occurs quite quickly, in the order of hundreds of milliseconds.

A subtle weakness with any belleville stack regulator can be seen in the final end state when the valve should be fully closed, at which point the regulator should be balanced with pressure on one side of the piston generating sufficient down force to counter the opposing belleville washer force... enough to fully close the air valve. However, in practice the fact that the two forces are quite closely matched leaves the air valve precariously balanced and provides scope for some level of valve movement, especially over time. This is what causes regulator creep. A good design for both the piston and air valve will go a long way to reducing or even eliminating creep and some manufacturers (such as the very skilled Robert Lane) go further by using coil springs instead of belleville washers to reduce creep and to improve regulator responsiveness. A downside with coil spring designs is that the valve body will generally by longer compared to a similar Belleville washer design, but a spring also has a smoother, faster and more consistent reaction movement characteristic. The problem with the Artemis design is that the air valve is horribly leaky - no matter how much fettling is done to try to improve matters. If you wait long enough the pressure on both sides of the air valve eventually equalises. In some ways it behaves more like a controlled leak... which was a huge disappointment to me because in all other regards the PP700 is a really exceptional little air pistol for sports and competitive use. The problem for a sports shooter with this arrangement is that the regulator behaves differently depending on the time interval between shots. If that interval is the same every time then the regulator will generate almost identical pellet speeds each time a shot is taken. However, (and back in the real world) if the interval between shots varies then the pellet flight speed will vary horribly and with consequential poor grouping. Tests confirmed that a visible difference to the point of impact at even 15 yards would occur if the interval between shots varried by as little as 20 seconds. It isn't easy to improve this, but one workable method for using the PP700W for competitive use is to disable the controlled leak regulator valve altogether and instead use the pistol between two bounds of pressure (for my particular arrangement 130 BAR down to 90 BAR). So long as the diameter of the transfer port in the rotating breech is reduced, the result is around 20 accurate shots and where at the absolute peak (around 110 BAR) the muzzle energy can be set just below the UK permitted limit. This arrangement works well and is very safe... because even if the pressure drops below 90 BAR or is higher than 130 BAR, the muzzle energy will remain much lower than the legal limit. The only real disadvantage is that after only 20 shots you are forced to refill with exactly 130 BAR of high pressure air.

Around 3 years ago Huma-Air released an aftermarket regulator for the PP700W - based on precisely the same type of Belleville stack arrangement as SMK or Artemis... but this time employing a properly engineered and effective air valve. The Huma-Air designed regulator fits inside the main high pressure reservoir and so does reduce the overal reservoir volume, but on the other hand... it works. I fitted one in my .177 calibre PP700W a few weeks ago and now instead of just 20 shots and the irritation of having to constantly refill the main reservoir to 130 BAR, I can fill the main reservoir to 220 BAR and obtain 50 to 52 full power shots while sports shooting on the range.

For anyone thinking of tackling this modification, I can tell you that it doesn't involve any difficult engineering other than the ability to safely strip down the PP700 (see the Huma-Air fitting instructions here). If like me you happen to be a UK resident, make sure you mention this to the Huma folks at the point of ordering because UK power levels for air guns are often much lower than can be used elsewhere in the world and so the regulator Huma sends will need to be appropriately calibrated. After now using the PP700W on the range for bell target practice, I can vouch for the fact that the Huma regulator is a very effective improvement for the PP700. Comment | Back to Quick Links...