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Question New chrono!

Discussion in 'Anything Airgun Related' started by sn1per, Mar 7, 2012.

  1. sn1per

    sn1per Engaging Member

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    Got my chrono (cb625) in the post today! I decided to see what my s400 is doing, in 16 shots between 10.9 and 11.4? I weren't on a full fill as I only fill to 170bar, is us right for power to fluctuate this much? I happy for it to fluctuate a little but not this much :(
     
  2. john2667

    john2667 Engaging Member

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    You need to look at the FPS readings rather than the FT/LB, depends on what pellets your using and the weight of them
     
  3. Irvysan

    Irvysan Posting Addict

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    If i read your post correctly a 0.5 ft/lb spread is not a lot, is the gun regulated? what calibre? you should go with the FPS as stated, if you need the manual its here : http://www.airgunforum.co.uk/forums/airgun-data-center/124347-combro-manuals.html

    you can go onto the combro site and check the calibration.

    HTH

    Irv
     
    Last edited: Mar 7, 2012
  4. john2667

    john2667 Engaging Member

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    Whats important is the consistancy of the FPS it shouldnt really be anymore than 10FPS max at the most between shots. What pellets you using and what calibre
     
  5. sn1per

    sn1per Engaging Member

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    Hi guys

    It's a .177 and I'm using air arms diabolo 8.4 grain.
    The rifles not regulated......yet

    If I get chance tomorrow I'm going to fill up and empty the full cylinder through the chrono at the range, see what's happens then.
     
  6. sn1per

    sn1per Engaging Member

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    What's the difference with the fps and ft/lbs? Are they not directly related? And what's the legal limit when using fps? Thanks.
     
  7. Gizza22196

    Gizza22196 Guest

    the fps is part of the calculation used in working out ft/lb
    the equation is : fps x fps x pellet weight (grains) / 450240 =
    or something along the lines of 598.2 x 598.2 x 14.5 / 450240 = 11.524358075693
     
  8. Accuspell

    Accuspell Major Poster

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    If you do, write each one down as you go along. You will need an A4 pad of paper to do it. Even better, note down the pressure (you'll have to go with the crap AA gauge) for each 10 shot group. Start at your full fill, write down the pressure and the FPS for each of those shots (it is 1 magazine at a time you see!). Write down the pressure at the start of the next 10 shot group and the FPS for each of these 10 shots. Do that for the entire fill (about 140 shots all told? 150 maybe) I wouldn't let it drop below 100 bar, you know it is crap by then anyway!

    Look at your results. If you do it neatly you'll probably notice the most consistency occurs when the pressure is between 180 and 120 bar. That should give you 6 magazines of pretty consistent shooting- you might squeeze another 5 shots from each end, but you would be better to fill her up instead. When filling to shoot, only fill to 180 bar, then you know you are in the good sctor from shot 1 to shot 60....then fill up instead of having a few dodgy ones.
     
  9. Dunkman

    Dunkman Posting Addict

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    Just a 12 fps spread will give you a varience of .5 F/lbs with Superdomes in .22. Consider that as a % of FPS variation this is only 3% which is nothing.

    This is another reason why running any air rifle over 11.5 F/lbs puts you precariousely close to the legal limit for non FAC.

    There is no legal limit for FPS. Roughly speaking it is about 575FPS for a .22 weight pellet but up to around 800 FPS for a .177 weight pellet so as you can see, the weight of the pellet is a deciding factor along with its velocity. Slow and Heavy can be as powerful as light and fast. What one lacks in weight it is compensated for by it's velocity to achieve the same muzzle energy.
     
    Last edited: Mar 7, 2012
  10. Irvysan

    Irvysan Posting Addict

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    the fps is feet per second which is the velocity

    ft/lb is foot pound or f.p.e which is the energy

    they are not directly related per say as the weight of projectile comes into play. (the pellet)

    an example :

    my rifle produces 11.19 fpe which is approx 593 fps with a 14.3 grain pellet (.22)

    Your rifle when producing 11.41 fpe with an 8.4 grain pellet is approx 782 fps (.177)

    so as you can see the fps is considerably higher with your calibre

    in order to acurately measure the power the fps is required as well as the weight of the pellet.

    I use this site : http://extorian.co.uk/shooting/pellets.html to calculate

    and I have a combro too but set to show fps NOT ft/lb

    HTH

    Irv
     
  11. sn1per

    sn1per Engaging Member

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    If I don't get there I'll have to build a pellet catcher of some description instead of the lawn, to do it in garden :)
     
  12. Dunkman

    Dunkman Posting Addict

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    The pellet catcher in my workshop is a large plastic bucket filled with rags. Stops the pellets and no impact noise. The old jeans in the bucket are a bit 'holed' now though:)
     
  13. Accuspell

    Accuspell Major Poster

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    He's right though. Velocity is actually what we are limited to.

    The Law states a maximum energy level, but that actually means a certain pellet is limited to a maximum velocity. The higher the velocity, the less pronounced the arc of flight. You can gain velocity without going over the limit by using lighter pellets.

    I know everyone goes on about legal energy level, but that is a flat line at the top. Underneath that flat line is a whole range of possibilities with different pellets. Once you know which is your best pellet, you can work to its maximum velocity to stay legal. It is just a reworking of the formula.

    For the 8.4gn Air Arms fields the enquirer is asking about, the maximum permitted velocity is 800fps - bear in mind though, if you get an 8.5gn one that does this speed, you are over the top!

    8.4 at 800fps = 11.94 ft-lbs
    8.5 at 800fps = 12.08 ft-lbs

    So you can see, you don't have a lot of leeway once you are close to the limit, which is why most people give themselves a bit of breathing space,or margin for error and set their rifles for a bit less than this.

    8.4 at 790fps = 11.64 ft-lbs
    8.5 at 790fps = 11.78 ft-lbs

    So, you can see why I say it is more practical to think of the limit as one on velocity, rather than one on energy. Simply learn the maximum permitted velocity for various pellet weights and you don't have to do the calculations more than once! Here I have done the one for AA fields in .177 for you already! You don't have to know them all, just the ones that matter to you for your rifle. If you don't have a certain pellet, you can't test it in your rifle anyway. Quite what your rifle willdo with a certain pellet is then open to conjecture, and can perhaps be estimated through experience and other peoples' experience of it through similar powered rifles. Provided you don't sail too close to the wind, you'll be fine.

    So you see, the examples above are a bit too close. Take the velocity down just 5fps to 785 and there is more margin built in.
     
    Last edited: Mar 7, 2012
  14. Dunkman

    Dunkman Posting Addict

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    Good points and also just goes to show how the current law stating a max of 12 F/lbs WITHOUT specifying a Calibrated 'test' pellet really is a can of worms. If the law said "you cannot exceed a max muzzle energy of 12 F/lbs with a home office approved 'pellet X' then wouldn't that make life so much easier for one and all. Anything over 11.5 F/lbs really is sailing potentialy close to the legal wind.
     
  15. Irvysan

    Irvysan Posting Addict

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    Thats what i was trying to get at but i was just typing what was coming out my brain. the ft/lb is only half the story. the more interesting part is the projectile weight and FPS.

    the velocity and weight are the deciding factors NOT the ft/lb which is just a calculation based on those 2 factors.

    sorry im rambling again............

    Irv
     
  16. Bwarnos

    Bwarnos Busy Member

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  17. sn1per

    sn1per Engaging Member

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    So I want it to be about 780-ish?, as I know that this rifle is a lot more accurate with the AA's and this would give me energy around 11.3, it's all a balancing act I guess!!
     
  18. Accuspell

    Accuspell Major Poster

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    Bwarnos - did you weigh your pellets so that they were all the same weight, not nearly, but exactly to the nearest .01 of a grain? I have a set of jeweller's scales that will weigh to that accuracy (not expensive, about £30). By weighing out a batch of pellets and using the ones all the same weight, in the middle of the variance (median average) you then remove one of the variables. The other pellets are fine for using, but when testing you want to use all the same weight ones out of the tin. Even the best pellets have a slight variation within the tin, normally only .1gn either side of the weight stated. I have just weighed a few Barracudas, the STATED weight on the tin is 13.27grains. If you went with this and set your rifle accordingly without weighing the pellets you would be in hot water for sure, even if you called them 13.3gn, becaue they actually weighed 13.7gn, or 13.6 or 13.8 and there is a misprint on the tin!

    On a pellet of 13.5gns a difference of .1gn is actually 1/14th, or about 6%. How much did the figures you obtained vary by, on each 10 shot string? Less than 6%, so if you tightened up on th eactual pellet weight in those strings you variation of results would be squeezed much tighter and be much more accurate. For "out of the tin" results they are fine. If you have a tin of pellets that are as far out from stated weight as these were, you are right on the upper limit of legal, because you plugged in 13.3gns, but the pellets were actually 13.7gns - the velocity was the same as it actually was though. Do the sums and see how much extra power there might be, purely down to a misprint of the pellet weight. This is just an example because I don't know what pellets you were actually using - in.22 somewhere arond the 15 mark though for certain.

    A general rule of thumb for wrking out legals with various calibres is Maximum velocity for:-

    .177 around the 780 fps

    .20 around the 640 fps

    .22 around the 590 fps mark

    these all depend upon individualpellet weights, but they are a ball park figure with realistic pellet weights for the calibre. A light weight pellet for the calibre will go a lot faster, of course, but the average type of pellet for the size, these figures will flag up alarm signals if your results are significantly different. Use a specially heavy pellet in a particular calibre and get near these velocities and you're in trouble, but you know when you have a heavy pellet. For the figures I used 8.5gn, 13gn and 15gn respectively. As I said, the right kind of weight and velocity for each calibre.
     
  19. david000

    david000 Member

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    Accuspell - good posts.

    I'm tempted to adjust the power on mine from it's current 11lb to nearer 11.5 lb - That extra 4.5% is appealing, but I need to stay legal.

    I hadn't considered weighing pellets, mainly as I don't have accurate scales, but given your Barracuda example it's obviously worth doing. Do you know if it's possible to buy 'calibrated' pellets - pre weighed for testing ?

    Also, how long a string would you shoot to confirm the correct setting ?
     
  20. sn1per

    sn1per Engaging Member

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    If the pellets are out on weight surely the manufacturer should state weight between 13 > 14 gn? As like stated above the slightly heavier pellets could put a rifle pushing high fps to the limit or even beyond? It feels like you have to weigh each one just to be sure!
     

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