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Air Gun Home Forum Index » General Air Gun Questions and Topics » Air Rifle Tech    View previous topic :: View next topic  Air Rifle Tech   Posted: Sat Nov 06, 2010 12:29 pm ClickStop New Member Joined: 06 Nov 2010 Posts: 4 Hello Everybody, I am new to the air gun world and I have a few questions I would like to ask regarding the technology of a springer rifle. 1) How does the combination of diameter and strock of the power piston affect pellet velocity. And what is a typical ratio of power cylinder volume to barrel volume? 2) I read on a different post that several people are cutting down the piston to increase stroke. In that post someone said that they cut it too much and the piston was too light. The question is, why is a heavier piston desireable or not. My thoughts are that the lighter the piston the faster the acceleration of the piston. 3) What is the average compression ratio of a sring powered rifle in pounds per inch compressed? Thank you for your input. P.S. I am in Canada and we cannot have a fps greater than 500. I am just curious about the technology. Re: Air Rifle Tech   Posted: Sat Nov 06, 2010 3:30 pm Dukemeister Member Joined: 08 Feb 2010 Posts: 92 Location: Ontario, Canada ClickStop wrote: P.S. I am in Canada and we cannot have a fps greater than 500. I am just curious about the technology. Curiosity can be a good thing... however, to clear up the misconception presented in the above, Canadian firearm law does not have ANY resticttion on either muzzle velocity or energy. What the law states is that airguns with both muzzle velocity exceeding 500 fps and muzzle energy exceeding 4.2 fpe are "firearms" in accordance with the firearms act and therefeore the owner must be licesned (hold a PAL) and the gun must be registered. The flip side to the law is in fact a consession to casual Canadian plinkers, and that is guns with muzzle velocity less than 500 fps, regardless of muzzle energy, are not considered firearms and can be owned without a licence or registration. You will often see advice on this forum against modifying non-Pal guns to shoot at higher velocities > 500 fps unless the owner has a PAL for this reason. The offence of not having a gun registered is less severe than the one of posessing a firearm without a lisence. _________________ Duke ))))----//----------==   Posted: Sat Nov 06, 2010 3:47 pm kanyon Silver Status Member Joined: 09 Aug 2008 Posts: 1582 Location: New Zealand Hi.. the piston can be to light... relate it to what happens at the other end of the gun.. light pellet traveling fast can produce LESS enagy than a heavy pellet traveling slower...same with light or heavy pistons "traveling mass" I have looked at giving a gun more piston travel... but there is not much extra to be had before the seal entres the cocking slot.. _________________ Career Infinity PCP .22 Webley Patriot .25 Hatsan mod 75 .22 Hatsan mod 99 .177 (Quattro & SAS) Kral underlever .177 ............................................. .   Posted: Sat Nov 06, 2010 5:45 pm ClickStop New Member Joined: 06 Nov 2010 Posts: 4 Dukemeister: Yes when I posted that I understood the law, I just put it out there as a simple reminder for people responding outside of Canada. P.S from the RCMP website: 2. Air guns that meet the Criminal Code definition of a firearm, but that are deemed not to be firearms for certain purposes of the Firearms Act and Criminal Code. These are air guns with a maximum muzzle velocity of 152.4 meters or 500 feet per second and/or a maximum muzzle energy of 5.7 joules or 4.2 foot pounds. So I think the muzzle energy does matter. Anyways, thanks for the clarification it is nice when people are on top of the nitty gritty legal issues. Back to the tech: If the spring was sized correctly and it provided the required force on the piston at any given point in the cylinder, isn't the role of the piston simplified to just providing a seal? This goes back to what is the desired ratio of compression cylinder volume to barrel volume as to not waste air after the pellet has left the barrel, but still providing enough to have the pellet actually leave the barrel.   Posted: Sat Nov 06, 2010 6:23 pm rsterne Moderator Joined: 15 Jan 2008 Posts: 2998 Location: Coalmont, BC Quote: for certain purposes of the Firearms Act and Criminal Code. That is where the AND/OR applies.... for registration purposes, the gun must be over 500 fps AND 4.2 FPE.... but back to your original questions: 1. More air chamber volume basically means more pellet velocity in whatever caliber you are looking at.... There is no "typical" ratio of power cylinder volume to barrel volume.... although there may be an optimum.... Most springers produce a bit more energy in .22 cal than in .177.... However, the B-28 (a magnum springer) is more powerful in .22 cal than in either .177 or .25 cal.... On the other hand, increasing the transfer port size in a .25 cal might just change that.... ie that could be the factor that is limiting the power in the .25 cal.... just a theory.... 2. Shortening the piston to increase stroke is simply a matter of getting back the velocity lost when the gun was modded to Canadian non-PAL specs.... it does not appy to most "properly designed" springers.... they already have the maximum reaonable stroke length for their cylinder size and design.... Weight is another matter.... There is definitely an optimum piston weight for a given design of springer.... just like there is an optimum transfer port size.... The problem is that the "optimum" is influenced by a myriad of factors.... some as small as just the pellet weight.... 3. The "compression ratio" would usually refer to the swept volume of the cylinder divided by the volume of the chamber in front of the piston at the instant just before the pellet starts to move.... I'm sure there is an optimum, but not for us mere mortals to understand.... When you speak of "pounds per inch compressed" are you talking about the cocking force?.... you lost me there.... Your idea about barrel volume does not apply very much to a springer.... The piston reaches the end of it's travel and begins its rebound not long after the pellet starts to move and long before the pellet leaves the barrel.... That is why barrel length has very little effect on the muzzle velocity in a springer.... Bob _________________ Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! Airsonal: Too many to count! Swept volume, power, speed, and whatnot   Posted: Sat Nov 06, 2010 7:29 pm Dukemeister Member Joined: 08 Feb 2010 Posts: 92 Location: Ontario, Canada Got it about the law, just don't want the world to think Canada doesn't permit >500 fps airguns. So on to the real questions... Note, I’m a newbie myself and this is my understanding (or misunderstanding) of spring piston rifles which I have become very intrigued with and although I’m enthusiastic and passionate, I may be somewhat clueless… anyhow here goes.. A spring piston rifle is designed to propel a pellet via adiabatic compression then expansion of a fixed volume of air. The parts of finely tuned rifle all interact with one another producing a consistent energy release every shot cycle, unless of course something is wrong with the mechanism. For a given design, the only real fixed part is the diameter of the compression tube. The volume of air available for compression is a function of the diameter of the tube (area really) and the length of the stroke. Generally a larger swept volume would lead to greater air mass and energy output, but because of interaction between the various parts, there will be an optimum swept volume for each rifle. But, where does the energy come from? Conservation of energy dictates that with the absence of fuel, the maximum available energy must come from the energy stored in the compressed spring. Theoretically, if everything was perfect and you had a frictionless mechanism without leaks and an ideal gas, you could get all the energy from the spring into the pellet, but of course in reality there will be losses. So, there are three main areas to concentrate on to optimize power output: 1. Increase the energy stored in the spring. The energy a spring stores is E=0.5FL, where F is the spring load and L is the amount the spring compresses. A longer spring and cocking stroke can compress more and give more power than a shorter one; also a stiffer spring produces more force for a given compression and will store more energy. 2. Reduce Friction. Friction will consume some of the energy when the spring is released, and every contact surface is involved. This is where materials and lubes come into play. 3. Optimize the swept volume. This allows the spring energy to be transferred trough compression/expansion of the air mass into kinetic energy in the pellet. Other considerations include: size of transfer port, friction of pellet in the barrel, pressure losses through breach seals, etc, that also play a role in robbing energy from the pellet. The mass of the piston plays a role, since the piston energy is 0.5MV*V where M is the piston mass and V is its speed. Empirical evidence shows that there is a optimum piston mass for a given system. Note that there is no mechanical link between the piston and air column, so in reality the spring energy must become piston kinetic energy to compresses the mass of air. If there is fuel in the system, the heat generated by the compression of air can ignite it generating more energy and increasing power. If this happens in the air chamber it is called detonation and can wreck the piston and spring and even blow up the compression tube. If it happens in the barrel it's called dieseling and can produce inconsistent energy, reducing accuracy. Some high power spring air rifles supposedly rely on these fuel burning techniques to achieve their rated power. So in the end, you have a complex system, the spring being at the heart of it. When a springer is "tuned" you see a variety of springs, seals, lubes being applied to fine tune the energy developed and transferred and minimize the banging and twanging of the internal components. Spring rifles detuned to meet market demands often use cut down springs, lower swept volume (longer piston) or built in leaks to reduce the energy transferred to the pellet. The shortened piston mod the OP refers to is for the Quest/Phantom and other B-18 based rifles sold as non-PAL in Canada. The stock full power Phantom/Quest produce a nominal power output at the muzzle of about 12-14 fpe. Crosman used a extra long piston giving a shortened cocking stroke compared with the full power versions on the non-PAL rifles that develop about 5.5 fpe. I learned on here that they used a shortened spring in the detuned .177 calibers as well, but only used a modified piston in the .22 versions, i.e, .22 calibres have te full power springs. Some .22 versions have a vent hole in the piston instead of the longer piston. With the longer piston I think there are 2 mechanisms in play for detuning: the spring stores less energy because the cocking stroke is shorter, and the air volume/mass is reduced. If you try to make the piston smaller than the stock full power piston then the seal ends up in the cocking slot which is no good. Or if you make one out of lighter material like titanium, you might get a bit more power, but again maybe not. To get more power, you step up to the Superstreak, or B-22 which has a bigger diameter tube, a badass spring, and other tweaks and it puts out 25 fpe and kicks like a mule. ... and what Bob said! I just read his post, It took me about 2 hours to write out the above diatribe. so I must post it anyhow. _________________ Duke ))))----//----------==   Posted: Sat Nov 06, 2010 8:21 pm rsterne Moderator Joined: 15 Jan 2008 Posts: 2998 Location: Coalmont, BC Quote: with the absence of fuel.... If there is fuel in the system.... And therein lies the rub.... Cardew claims that all springers with the exception of those operating in what they term the "pop-gun" mode get a significant portion of their energy from combustion of the thin layer of lube left behind on the walls of the compression tube by the seal during the previous shot cycle.... I have no idea how this produces a consistent shot velocity.... but they certainly have a valid argument because they tested it in a Nitorgen atmosphere and lost (IIRC) about a third of the power.... It would seem that serious target rifles are tuned to operate in the "pop-gun" mode while all medium and magnum springers use combustion to increase the power they produce.... Again, Cardew claims that it is combustion, not detonation that is occurring under normal circumstances.... although using too heavy a pellet may cause an otherwise normally functioning springer to detonate and damage components such as the spring.... Some tuners claim to be able to recognize the damage caused by using heavy pellets.... I'm currently in the camp that have no idea whether Cardew are right or not.... I have read about experiments where springers were cleaned and fired with no lubricant with very little loss in power.... Those who consider Cardew's book to be the bible on springers simply dismiss those experiments, claiming that some lubricant remained behind.... The biggest problem I have with the combustion theory is understanding how a springer can be so darned consistent over thousands of shots without adding any lubricant.... and yet they are.... Bob _________________ Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! Airsonal: Too many to count!   Posted: Sat Nov 06, 2010 10:17 pm ClickStop New Member Joined: 06 Nov 2010 Posts: 4 Quote: 3. The "compression ratio" would usually refer to the swept volume of the cylinder divided by the volume of the chamber in front of the piston at the instant just before the pellet starts to move.... I'm sure there is an optimum, but not for us mere mortals to understand.... When you speak of "pounds per inch compressed" are you talking about the cocking force?.... you lost me there.... Well two things here. First I meant that as the rating of the spring. I was wondering how many pounds it takes to compress the spring one inch. (without the advantage of the leverage of the barrel while cocking) Just as a real world example here, my car springs are around 300 lb/in. And you kind of hit on the other one there, what are some approximate air pressures seen in a springer rifle? Obviously they will vary depending on the rated fps, but just a general value is good. Are we talking 50, 100, 200 psi? Also does anyone have some approximate values on the piston diameter and stroke for various models? Back to this piston and cylinder disscussion, just for fun here lets go to the extremes of the spectrum. If you were to use a piston (assuming piston mass stays constant in both situations) with the same area of the barrel and a really long stroke. Against a very large area piston (again with the same piston mass) and short stroke, and lets say both cases have the same swept volume and spring rate (force) behind them, what would be the difference in rifle performance. Also, just from reading around it sounds like there are a ton of companies copying designs off of one another. Just an interesting comment. Thanks everyone for the comments and replies.   Posted: Sat Nov 06, 2010 10:40 pm rsterne Moderator Joined: 15 Jan 2008 Posts: 2998 Location: Coalmont, BC I have no idea what the spring rate is.... Most of the springs are under preload.... and the magnum springers are under a LOT of preload.... I have heard the number 1300 psi several times regarding springers.... have no idea how accurate that is.... I would imagine that the magnums would run more than that for peak pressure.... Mainsprings start at about 1/2" ID and 3/4" OD.... at least I haven't seen any smaller than that.... so I'm guessing that the cylinders start at about 7/8" ID.... AFAIK, the B-26 has either a 25 or 26mm cylinder (about 1").... My B-28 is AFAIK 28mm (1.1") and it is a magnum springer.... I am sure that part of the reason for choosing cylinders about an inch in diameter is for the appearance and ease of production and cocking of the gun.... It would not be possible to have a cylinder of barrel diameter and many feet long as you could not carry it or cock it.... At the other extreme, if the piston was 6 inches in diameter and 1/2" stroke the spring would be massive and heavy, the cocking linkage would be massive.... and the gun would be ugly and very heavy.... Let's face it, only a certain range of sizes is practical for an airgun.... Bob _________________ Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! Airsonal: Too many to count! springs and pistons   Posted: Sat Nov 06, 2010 11:25 pm Dukemeister Member Joined: 08 Feb 2010 Posts: 92 Location: Ontario, Canada Bob has the diameters right. Most low/mid power springers seem to have about 25mm diameter ID compression tube. Magnum springers, B28, RWS 350 are about 28mm. As for spring rates, I saw on another forum it was about 48 lb/in for a B18 spring. Does anyone know what preloading a mainspring actually does? I will start a new thread... _________________ Duke ))))----//----------==   Posted: Sat Nov 06, 2010 11:47 pm rsterne Moderator Joined: 15 Jan 2008 Posts: 2998 Location: Coalmont, BC Preloading a mainspring increases the force on the piston.... If you have a 48 lb./in spring with no preload and your cocking arm moves it 6" the starting force is zero and the force fully cocked is 6x48 = 288 lbs.... If you have 2" of preload, the starting force is 96 lbs. and the force fully cocked is 384 lbs.... Pretty easy to figure which gun would shoot harder.... Shimming the spring increases the preload.... but eventually the spring will go coil bound before the sear engages in the slot in the piston.... The most power for a given spring is just before you get to that point.... where the guns cocks 100% reliably every time.... and won't fire when bumped hard.... Bob _________________ Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! Airsonal: Too many to count!   Posted: Sun Nov 07, 2010 11:04 pm ClickStop New Member Joined: 06 Nov 2010 Posts: 4 Thanks for the help everyone.   Posted: Mon Nov 08, 2010 7:04 pm Kazman Member Joined: 08 Nov 2010 Posts: 25 ClickStop wrote: Quote: 3. The "compression ratio" would usually refer to the swept volume of the cylinder divided by the volume of the chamber in front of the piston at the instant just before the pellet starts to move.... I'm sure there is an optimum, but not for us mere mortals to understand.... When you speak of "pounds per inch compressed" are you talking about the cocking force?.... you lost me there.... Well two things here. First I meant that as the rating of the spring. I was wondering how many pounds it takes to compress the spring one inch. (without the advantage of the leverage of the barrel while cocking) Just as a real world example here, my car springs are around 300 lb/in. And you kind of hit on the other one there, what are some approximate air pressures seen in a springer rifle? Obviously they will vary depending on the rated fps, but just a general value is good. Are we talking 50, 100, 200 psi? Also does anyone have some approximate values on the piston diameter and stroke for various models? Back to this piston and cylinder disscussion, just for fun here lets go to the extremes of the spectrum. If you were to use a piston (assuming piston mass stays constant in both situations) with the same area of the barrel and a really long stroke. Against a very large area piston (again with the same piston mass) and short stroke, and lets say both cases have the same swept volume and spring rate (force) behind them, what would be the difference in rifle performance. Also, just from reading around it sounds like there are a ton of companies copying designs off of one another. Just an interesting comment. Thanks everyone for the comments and replies. Hi . I just changed the pistons and springs in both my rifles . One being a Phantom , the other being a Benjamin Clasic. I'm just wiating for my Chrony to see how much difference . Even without the Chrony , i know these rifles are packing some serious power . The Phantom is the more powerful while the Benjamin is Deadly accurate .The difference in the pistons is very noticeable   Posted: Thu Nov 11, 2010 6:33 pm packrat Senior Member Joined: 16 Nov 2009 Posts: 882 Location: Out in the woods...Ellabell, GA. You DID shoot them over a Chrony before doing the changes, right? I see you mentioned that you're waiting on chrony.. _________________ HEY !! This getting old thing is NOT for sissies... 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