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Thread: The beginner's guide to air rifle power

  1. #1
    Sharp Shooter
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    Default The beginner's guide to air rifle power

    The beginners guide to air rifle power.

    So we buy our first air rifle or shop for our first air rifle. One of the things we want to know is how powerful this rifle is. The descriptions we often hear are:
    It’s a 1000 foot per second rifle.
    It’s a FAC rifle.
    It shoots right through a chipboard backstop.
    It shoots a ‘snotskoot’.
    It sounds like a .22 rifle.
    It goes through a battery.
    It klaps through a coke tin at 50 yards.

    These statements are as vague as saying your car is the fastest in town when asked how powerful it is. You might live in a town where nobody is interested in dicing you, even though the mayor drives a Lamborghini. (We live in SA, it’s possible, and he might have two.)

    Let’s look at a meaningful measurable standard to express air rifle power with.
    The 1000 fps rifle expression is only half true. It shoots WHAT at 1000fps will be the question?
    You get to a ten pin bowling alley, your friend challenge you. There are only two bowls, one normal size and the other the size of a big marble. You have to knock 10 large wooden pins down. The oke choosing the marble sized bowl will be able to roll it at great speed, but if it gets to the front it will simply bounce off the first pin it crushes into. The normal sized bowl will not have great speed when launched, but will smash into the first pin with enough power to knock 10 pins down in total.

    So we need a power expression that allows for both speed and weight of the thing travelling at given speed.

    We use either “Fpe”, foot pound energy abbreviated, or” J” for Joules. Joules is not as often used in air rifling but equals 1.35581795fpe.

    Ft-lb is the scientifically correct abbreviation. In air rifling we just refer to fpe. The ‘e’ we add to ‘fp’ so it does not get confused with foot pound torque which is a cross product of force and displacement vectors. Fpe is the energy needed to displace one pound by 1foot.

    We air rifle shooters understand each other when talking fpe. How do we get to this fpe expression of power?

    We take the weight of the pellet we are shooting with in gn (grain, not to be confused with g for gram), then we measure the velocity (speed) of the pellet in feet per second (fps)just as it exits the barrel, known as muzzle velocity.

    We apply these two measurements to this formula: (FPSxFPSxGrains)/450240 = fpe

    So we also need a calculator, well I do when I run out of fingers. There are on-line calculators to do this with or the handy ballistic program called Chairgun can also be used.

    How do we measure the pellet velocity?
    We use a chronograph or ‘chrony’. These devices vary in shape but all work on the same principle. The pellet passes two censors or light beams a known distance apart with a timer to measure the time it takes the pellet to cover the known distance. The chrony then calculates the speed the pellet needed to travel at to cover that distance.
    “I drove down to Durbs in 4 hours from Johburg” tells me you did low flying since I know Durbs is 600kms from Johburg.


    A 'sky screen' chrony. Dependent on ambient light to work. Rifle must be kept parallel and at 90deg at same distance for every shot to be accurate. Can be used for arrows and fire arms as well as pellets.



    Combro CB625 pellet chrony. Interfaces with laptop to record and calculate shot strings and energy values. Not light dependent, taped in place on silencer or barrel. Not suitable for fire arms, heat and powder blast will damage it.



    How do we know how many grains the pellet weighs?
    We either look on the pellet tin lid or if it’s pellets handed down from a china in a matchbox we weigh them using a pellet scale, also used by reloaders for weighing gun powder. So the bathroom scale won’t crack it, we need at least a tenth of a grain resolution so we can say the pellet weighs 8.4gn. Even better if we could say it weighs 8.44gn but a tenth scale will suffice.

    Applying the fpe knowledge.

    Ok, by now it is clear that we could use say a 12fpe rifle to shoot a very light pellet of 5gn and it will travel at 1039fps because the pellet is so light. If we use the same 12fpe powered rifle to shoot a ‘normal’ 8.4gn pellet with it will only exit the barrel at 800fps. A ’heavy’ 11.3gn pellet will only achieve 690fps from the same rifle. It makes more sense to say we have a 12fpe rifle than saying we have a 1000fps rifle that can shoot a feather at 1600fps.




    How powerful is a 12fpe rifle?
    We know it can shoot an 8.4gn pellet at 800fps but how does it relate to other air rifles? Let me categorise air rifle power roughly. I’m talking .177 or 4.5mm calibre here since it remains the most popular by far.
    6fpe
    Most popular pellets: 7gn to 8.4gn, 7.3 and 7.9gn most ideal.
    – Used for 3P 10meter precision shooting indoors. Rifles may be as advanced as the Anschutz 8002 or as basic as the CZ200T. Most PCP pistols, competition and other falls in this category. As for springers, small breaknecks like the HW30s and small Gecado’s fall in this category.

    12fpe
    Most popular pellets: 7.3gn to 11.3gn and 8.4gn most ideal.
    - By far the most popular and versatile category. This is also the maximum energy allowed in Britain for unlicenced air rifles. More powerful than this they need Fire Arms Certification, FAC for short. Because this is the most popular power class most pellets are available for this power level and most accuracy is achieved in this class with the largest variety of pellets. International Field Target competition rules this maximum allowable power and international Bench shooting Light Varmint class also dictates 12fpe.
    I say ‘most accuracy’. By this I mean groups the size of a R5 coin are possible at 50m.
    Rifles in this category are most varied, the Air Arms EV, Walther Dominator and many other top class FT raceguns fall into this category and so do most popular Sporter rifles including AA S400, Weihrauch HW100, Daystate Huntsman and many others.
    Springers in this category include classics like the AA TX200, Pro-Sport and Weihrauch HW77 and HW97’s.

    15fpe and 18fpe
    Most popular pellets: 8.4gn to 11.3gn and 11.3gn most ideal.
    These rifles are referred to as FAC rifles since they exceed the 12fpe FAC limit. Less popular in Britain and in South Africa. In SA because it is regarded as a ‘hunting’ energy level and here it is illegal to kill anything with an air rifle leaving these rifles to long distance plinking. The only ‘Protea’ sport using this power level is Bench rest and most bench shooters switch to .20 calibre when using these power levels. Dedicated rifles in this energy level don’t really exist. Some makers offer ‘souped up’ 12fpe versions to cater for this category and some shooters ‘super tune’ 12fpe rifles to achieve these power levels. The AA S400 and S510’s, Weihrauch HW100’s and Daystate Huntsman is offered as ‘FAC’ options.

    25fpe
    Most popular pellets: Anything that weighs more than 11.3gn and would shoot accurately. Due to recent development by JSB there’s basically only one option here, the JSB Monster at 13.4gn. Defiants, Rabbit Magnums, Eunjins and Pile Drivers are available but accuracy is dismal. Daystate has an offering at 15gn resembling the Defiants which I have not tested yet.
    Again only Bench rest territory as for sporting use. At these power levels the 5.5mm calibre is used most often, offering a wider choice of much heavier pellet types.

    What defines “most popular” pellet weights for different power levels?


    Accuracy is a relative concept, to some a 5shot 50mm group at 25m is accurate, others need pin point accuracy. These are 5shot groups at 25m, 12fpe, no wind.
    Pellet accuracy depends on velocity. If a pellet is too heavy for a given power level it travels too slow and becomes unstable in flight spoiling accuracy. If a pellet is too light for a given power range it will travel too fast and as it approaches the sound barrier it becomes unstable and inaccurate. Worse if it crosses the sound barrier. Pellet defects are also greatly enhanced at greater velocity. This is why pellet manufacturers cater for the most popular power levels and most pellet choices exist here in this weight range. In .177 calibre this is 12fpe.



    How do I make a rifle more powerful?
    This normally gets asked after the first ten shots were fired from a new rifle and accuracy gets questioned. This must be one of the biggest mis-perceptions amongst beginner air rifle shooters.
    In spring and gas ram driven rifles, referred to as ‘springers’ more powerful springs or gas rams are used to increase power. With this, recoil is increased resulting in less accuracy.
    In pre charged pneumatic rifles PCP's, more compressed air is used to up the power. With this comes a lot of design headaches. Longer barrels, rifle twist rate, hammer bounce, valve flow restrictions, port restrictions to name but a few. More power means less shots per charge, so you end up staying within 5 meters from the scuba cylinder.

    The biggest power draw-back:
    We have a dilemma in air rifling, caused by the limitations in pellet design. Many people have tried many designs over many years to make a pellet that could shoot both ‘fast’ and ‘accurate’ at the same time. It is not possible. The pellet must have a ‘skirt’ to seal against the barrel grooves and landings so the compressed air can push it.
    Because of this, unlike in fire arms where burning powder creates almost instantaneous enormous pressure, boat tail and jacketed designs cannot be used to stabilize the pellet.
    This is one of the main reasons we do not need fire-arm licences for air rifles. If we want to keep reasonable accuracy, we are forced to stick to certain power levels, no matter what we do.

    So what energy rifle should I get?

    This will depend on what you want to do with the rifle. Most ideal, like most shooters will tell you, would be 12fpe. Most air economic, most rifle choices and most pellet choices. Last but not least, useable for competitive shooting in both FT and Bench.

    Pellets vary greatly in design and weight. Experimenting will dictate eventual choice. Armed with some knowledge about power levels will help in deciding the ideal pellet for your application.

    Last edited by DvdM; 29-09-11 at 22:47.
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  2. #2
    Sharp Shooter

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    WOW! awesome guide!
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  3. #3
    Inactive Member

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    Easy to read, even easier to understand, great stuff, well done DvdM
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  4. #4
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    Very informative, should become sticky for all the noob's.
    As per usual, very well done Derek!
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  5. #5
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    This is great. Lets hope a newbie listens
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  6. #6
    Inactive Member

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    Very nice Derek
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  7. #7
    Sharp Shooter
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    Goeie werk !
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  8. #8
    Rest in Peace Dirk 08/2015

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    Deserves being a sticky, for sure.
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  9. #9
    Protea FT Team '09/'10
    National FT Champ '10

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    Nice print out for my Air rifle file.
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  10. #10
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    Thanx so much, just joined the forum today, so glad i did. First proper explanation i've read on all the issues worrying us noob's
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  11. #11
    Protea Benchrest (Air) Team '17/'19

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    Very very nice to see you on the forum again, Dave - welkom terug!
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  12. #12
    Protea FT Team '09/'10
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    Quote Originally Posted by JohanJan View Post
    Very very nice to see you on the forum again, Dave - welkom terug!

    Thanks oom Johan, maar ek kan nie elke dag kom nie, Perth is te ver.
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  13. #13
    Marksman

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    When I read

    We apply these two measurements to this formula: (FPSxFPSxGrains)/450240 = fpe


    My brain automatically rejected it, as i know E=VVM/2 or kinetic energy equals half of mass times velocity times velocity

    so where the hell did this 450240 come from

    After some research and scratching formulas on a scrap of paper the explanation of the constant 450240 is as follows.

    As a starting point from my high school physics we have (FPS X FPS X Grains)/2

    First as we are measuring foot POUNDS we need to get from grains to pounds to do this we divide by 7000, i.e. 7000 grains = 1 pound

    Now we have (FPS X FPS X Grains)/2 X 7000 or (FPS X FPS X Grains) / 14000

    hmm 14000 is still around 32 times too small, so back we go to the definition of Fpe which is 1 Fpe is the amount of energy to move one pound one foot

    what is often omitted from this definition is "upwards" or against gravity.

    remembering back to high school again gravity was about 9.8 M/SS Google tells me the conversion factor from meters to feet is 0.3048
    apply this to 9.8 / 0.3084 and we get 32.16F/ss

    okay so now we have (FPS X FPS X Grains) / 14000 fighting against gravity of 32.16 to do this fighting we divide by 32.16

    leaving us with (FPS X FPS X Grains) / 14000 X 32.16 or (FPS X FPS X Grains) / 450240

    who would of thought high school science would come in handy with a hobby 20 years later

    Thanks David for a great read and workout of the grey matter
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  14. #14
    Sharp Shooter
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    regbes, you actually REMEMBER all that from school?

    Here are two handy calculators.
    Airgun Conversions & Calculators - Air Rifle SA Forums

    Once you know your rifle's energy output you can also calculate velocity for a given pellet. Won't be as accurate as a chrony but will give you a good idea. Pellet velocity is also influenced by its BC, Ballistic coefficient, in short; wind resistance, a wadcutter (flat pointed) pellet will be slower than a more streamlined pellet of the same weight.
    BC is calculated using two chronys, one at the barrel and another a known distance away, say 25m. The rate it slows down from exiting the barrel to the next chrony determines it's BC.
    Again, the free software called "Chairgun" does this on the fly.
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  15. #15
    Marksman

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    Quote Originally Posted by DvdM View Post
    regbes, you actually REMEMBER all that from school?
    LOL once a nerd always a nerd

    Anyway you got me excited with those calculators so i made one in Excel

    I only have Excel 2010, but did try and make a file for older versions of excel.

    Hmm cant seem to upload the xlsx (2010 version) file, hope the save as older version one works

    Airgun Conversions & Calculators - Air Rifle SA Forums
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