?????  Airgun-Secrets  ?????     

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INTRODUCTION:

An air rifle is called an air rifle because it shoots with air. Looking a little bit closer it is compressed air which stores compression energy driving the pellet forward when expanding again. This definition is correct when applied to a precharged air rifle. In a spring piston-operated airrifle, however, the processes moving the pellet may be different but many airgunners do not realize that. In Great Britain an "Airgunner's Bible" was written by G. V. Cardew and G. M. Cardew (father and son): "The Airgun from Trigger to Target" which is, in my opinion, a MUST for every serious airgunner. The authors performed a tremendous amount of experiments with airguns which resulted in many novel, interesting, surprising and astonishing facts. One of their basic statement may be simplified in the following sentence: "a spring piston air rifle is a very complicated machine". At a first glance this is not easy to believe because the basic construction principle of a spring piston air rifle is relatively simple. Looking on its function, however, the involved mechanisms are much more complicated than one believes. Father and son Cardew called these phenomena in Their book as chapter 2 "The Four Phases". In my opinion the information and insights given there are interesting and important enough to deal with them here. I will not give a really detailed report because everybody with a deeper interest should read the book. I only will try to present the most important information in my own simple words as a summarizing report.

THE SECRET FOUR PHASES:

The basic statement is that a spring piston air rifle may operate in any one of four distinct phases which are the "blowpipe phase", the "popgun phase", the "combustion phase" and the "detonation phase".

The blowpipe phase is the operating principle of "weak" air rifles, in which a BB or a lead ball does not fit the barrel exactly. In fact, many BB air rifles or pistols have smooth barrels and the balls have to be fixed in them by small springs or magnets to prevent them from falling out of the muzzle. Because the barrel is not tightened by the ball the moving spring piston is not able to produce enough pressure to produce higher velocities. So the gun really functions like a blowpipe. This action principle is due for weak airguns and softair guns. In this context the blowpipe phase is not very important. Nevertheless, this principle may be very successfully applied as demonstrated by the blowpipe arts of Indians in South America.

The popgun phase may be described in that way, that a pellet tightens the barrel nearly perfect and that it is moved out of it solely by the pressure of the air created by the moving spring piston. This is the moment to grin and to ask "how else?". But exactly this is the point to watch up because there are in fact some other phenomena involved! It has to be pointed out, however that most high precision spring piston airguns for the 10 m shooting disciplines are operative in exactly this way. The comparison with a popgun is not totally exact because the plug is inserted into the muzzle and not into the opposite end of the barrel as the pellet is. All other functions are, however, identical: the spring piston compresses the air when moving quickly forward which then, at a certain critical point, starts to push the pellet forward and out of the barrel. Naturally, the air heats up in the course of the compression which results in a further increase of pressure. When the pellet, however, is then driven trough the barrel the air cools down behind it and all the additional pressure is lost. So, the pellet receives its energy solely from the forward moving spring piston. Also this kind of airrifle belongs to the "weak" ones but the popgun principle can be calculated perfectly and technically controlled.

The combustion phase describes the function principle of the most spring piston air rifles with a higher muzzle energy. When the piston is moved forward rapidly by the strong spring the temperature of the air in front of it increases with the pressure. This temperature reaches such high levels that lubricants or any other inflammable substances start burning. This results in a further increase of the pressure which drives the pellet through the barrel with a high velocity. Because this combustion is induced by the high temperature resulting from the rapid compression of the air father and son Cardew called this phase initially "Diesel engine phase". At a first glance this term seem to be a good comparison but it led to complications with the definition of the next phase which will be described later. In any case: following this function principle an airgun is a "real firearm" in which the combustion of the lubricant replaces the combustion of gunpowder. Again, at a first glance this is not easy to believe, but the experimental proof will be given that this is true. It is clear that the intensity of the combustion depends upon the amount of lubricant which can be combusted which results in some uncalculable effects. Therefore, these strong air rifles are less precise than those match air rifles working according to the popgun principle. This does not mean, however, that one should pour lubricant into the transfer port to obtain more power, because this may result in a damaged gun. It is well known that the muzzle energy of such airguns slowly decreases if they are not properly lubricated from time to time. This is no problem of sealing but a problem of the decreasing amount of combustible lubricant. Sceptic? Wait a moment!

The detonation phase is not easy to analyze because it is rare. If it happens, however, this may end up in a disaster and if induced experimentally mostly the airgun will be ruined. As far as this phase is understood it results from the fact that a certain crital excess amount of "fuel" is present in the compression chamber which induces a much stronger combustion with very high temperatures. The latter then obviously induce a chain reaction which is the detonation of all inflammable material at all. So, a large amount of energy is created. Now the reason for my warning not to fill lubricant into the transfer port is more than clear. Some crazy "experts" claim that a drop of ether in the compression chamber would increase the muzzle energy drastically. This is true but illegal and dangerous and one should not play such doubtful games. The more harmless case of a detonation phase is the "Diesel effect" of an air rifle which occurs if the air rifle was lubricated too well. One can detect this effect easily with the louder and sharper noise, a smell of burned lubricant and the smoke which leaves the muzzle. Sometimes also sparks are thrown out of the muzzle which are mostly burning down particles deriving from the piston seal. I the majority of cases the "Diesel effect" does not damage the gun but in strong and critical cases the detonation drives the piston back with a high force thus damaging the spring or itself. The detonation phase is the reason why lubricated pellets should not be used in spring piston rifles because the lubricant may be spoiled into the transfer port and explode there. The term "Diesel effect" is also in this case not correct because it describes an uncontrolled detonation whereas the processes in a Diesel engine are controlled.

These 4 phases must not necessarily take place separately but may occur in an airgun for short times in sequence. If, e. g., an air rifle was overlubricated the first few shots will be detonation shots which then change to combustion shots. If the rifle is not lubricated again after a longer time it max work then only in the popgun mode. If one uses a too small pellet which does not fit well into the barrel the airgun then will work in the blowpipe mode.

Sceptic airgunners now may think: "who believes it may believe it!" This obviously did also father and son Cardew and contemplated about a way to proof the combustion phase. The result was the so-called "Nitrogen Experiment". To perform this they took a Weihrauch HW 35 air rifle in caliber .22, took it down totally, degreased all parts thoroughly, assembled it again lubricating carefully and correctly all necessary parts and spots. After that it was "worn in" until a chronograph showed constantly a muzzle velocity of 636 Fps with a 14.4 grain pellet. This corresponds to a muzzle energy of 12.9 Ftp. Thereafter the stock was removed and the air rifle was packed in a plastic bag together with a sufficient amount of pellets which was attached for 30 minutes to a vacuum pump to remove all air. Then the bag was sealed gas-tight and filled with nitrogen which is an inert gas not allowing combustion. The barrel was outside the bag but the hole in the latter was air-tight sealed around the barrel the muzzle of which was closed by a rubber bang. Then a series of shots was performed in that way the the rubber bang was removed shortly before firing and then immediately inserted again. The result was impressive: the muzzle velocity was only about 426 Fps which corresponds to a muzzle energy of approx. 5.8 Ftp. Removed from the nitrogen bag and operated in the fresh air the good HW 35 easily developed again its original power. Convinced now?

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