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Question grain size vs velocity?

Discussion in 'Anything Airgun Related' started by jimmyhackers, Jul 15, 2014.

  1. jimmyhackers

    jimmyhackers Well-Known Member

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    hopefully this is a simple one

    been googling and i cant find a definitive answer....maybe there isnt one?

    im looking for a general rule of thumb if there is one.

    heavier grain = slower "exit" velocity is what im steered to thinking....

    but thats a very simple assumption on my part using the same gun and same/similar shape pellets but of varying weights.

    to put it into context i shoot from a p17 which is a single stroke pneumatic. i plan on getting a webley rebel which will be multipump pneumatic. so im asking from the context of pneumatic primarily.

    i do have a 22 springer rifle. do springers follow a differnt rule?

    thanks in advance
    jim
     
  2. warrenater

    warrenater Donator

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  3. Accuspell

    Accuspell Pro Poster

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    Think of it this way, if you pick up a light stone and throw it, and then pick up a heavy stone and throw it...which goes FASTER?

    The lighter the pellet the greater the velocity it can carry and stay within the 12ft-lb legal ruling. It follows, therefor, that the heavier the pellet the slower it has to travel to remain within the legal restriction.

    Now, with PCPs (or a pumped pneumatics, which are essentially the same thing, just with a pump built in, whether that is single stroke or multi-stroke) the rule of thumb you ask for is: heavier pellet = greater energy. There will be some pellets that haven't read the book, because there are then a few variables that can play a part, such as lead content of the pellet, head size and skirt deformation for sealing. BUT in general terms with a PCP you will get a higher power reading with a "heavier" (I put it in commas because there is a limit - fill the barrel with lead an dit won't work!) pellet than a lighter one....except for AA Fields! They haven't read the script and can throw a wobbly into the picture.

    Spring guns are the opposite, they tend to produce the most power with a "lighter" pellet.

    This is all on the proviso you don't go to extremes. With a (eg) 13gn - 18gn pellet variance in .22 this rule of thumb will work. Chuck in a 32 grainer and no, you won't get more power because it will be outside the bounds, unless you have the extra waft to drive it down down the barrel. Get that 32gn pellet up to 900 - 975fps and you would have a LOT more power! Within the 12ft-lb limit there will be barely enough waft to get it out of the barrel.
     
  4. smartarze

    smartarze Active Member

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    If thats the case, If the police were to test the rifle using their prefered choice of Bisley Magnums, how can it be a fair test? Springer would show a lower reading and a pcp would show a higher reading?

    If u had a springer shooting at 12.5 ftlbs on AA Fields it would be legal when tested with magnum's
    If u had a pcp shooting at 11.5 ftlbs on AA fields it would be illegal when tested with magnum's

    confused.com!!!
     
    Last edited: Jul 15, 2014
  5. r10hunter

    r10hunter Honorary Member

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    The police may use any pellet they choose.

    They will know full well that a light pellet produces more power in a spring gun normally and a heavy pellet more power in a pcp normally.
    They have plenty of time and a selection of pellets to try if the gun goes over with any pellet you have problems.
    Cheers Andy
     
  6. smartarze

    smartarze Active Member

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    ok cheers for clearing that up Andy, I just assumed they used only magnums as I've read it in many different places. Its still a bit of a lottery tho! Its impossible for us to try every single brand of pellet on the market to check it doesn't creep over the limit.
     
  7. r10hunter

    r10hunter Honorary Member

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    Yep!! That's why it's normally recommended to set your power at 11.5ftlbs with the most powerful pellet in your gun.
    Cheers Andy
     
  8. mattyts

    mattyts Donator

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    It's not that they use Bis mags,I think they actually shoot three heavy,three meds and three heavies then take an average,it's that in PCPs,most of the time a heavier pellet yields higher power so setting a gun up within the limit with a heavier pellet means it should be safe with others but it's not always the case.

    TBH,testing your rifle with every pellet is actually not as bad as it seems when you consider how many pellets are the same but branded up differently, I would try about 5-6 types then be happy if it's under with all of them.
     
  9. jimmyhackers

    jimmyhackers Well-Known Member

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    thanks, thats good food for thought....but i feel i still need to clarify

    so as is with my pneumatics, its actually kind of the opposite of what i thought.

    heavier = faster?
    or
    heavier = same-ish exit speed but more power?

    does that mean its safe to say in a pneumatic that the velocity remains resonably constant (within said grain limits)

    without a chrono im just speculating..... basically i want some pile drivers (for no reason except owning/trying the heaviest pellet about) buy the sounds of it police test with a variety of pellets.

    but say if they found piledrivers with my guns would they have to include them in their tests?
     
  10. r10hunter

    r10hunter Honorary Member

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    Ftlbs is worked out by (velocity of pellet X velocity of pellet X weight of pellet in grains divided by 450240) = ftlbs for that pellet
    Put some figures in the calculation and have a play you will soon understand.
    Heavier pellets will have more power at the same speed as a light pellet.
    Some pellets are able to utilise more of the air blast than others.
    If you have pile drivers on you they will probably be tested.
    Cheers Andy
     
    Last edited: Jul 15, 2014
  11. Clubshot

    Clubshot Clubshot

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    Piledrivers - even though 21.00 Grains in .177 - They do not always go over power in many gun's

    Some light weights will go over power in some guns

    Been well discussed - there is No ideal Test Pellets for all UK used Air Guns -

    BOB/R
     
  12. Greg1983

    Greg1983 Busy Member

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    When I tuned my rifle I was told to set my gun up to the desired ftlb I wanted using bisley magnums there for any other pellets I use that are lighter will not exceed it, and that's how i set mine up and for the police test don't they test the rifle using 3 different types/weighted pellets?

    Cheers, Greg.
     
  13. mattyts

    mattyts Donator

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    Yes,the thing is,that's a very broad and vague statement, usually, PCPs produce higher power with a heavier pellet and springers produce higher power with a lighter pellet but this isn't always the case so setting it closer to the limit with bis mags isn't always the best thing to do.
     
  14. Greg1983

    Greg1983 Busy Member

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    I see, well that's the method I have used to set mine and after doing so I tested some brands of other pellets through the rifle with chrono and they seem to stay below the limit but I'll take on the info when it comes to doing my PCP rifle :)

    Cheers, Greg.
     
  15. jimmyhackers

    jimmyhackers Well-Known Member

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    ive seen those little calculations..... and a few calculators that do it for you.

    problem is i cant tell what my pellet speed is untill i get a chrono. (very tempted to do so)

    the stated speed of my p17 is 410 fps i think....but ive seen many reviews and it seems to range from 350fps upto 410-420. so all my calculations are based around that max limit. so a piledriver 21 grain pellet going at 410 will be illegal at 7.84 ftlbs and close to illegal at 350fps being 5.71

    but will my p17 even get to those speeds firing one?it seems so heavy i cant imagine itd get much velocity into it.

    what do you guys think?
     
  16. mattyts

    mattyts Donator

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    I may have mis understood you but a gun doesn't spit out every pellet at the same speed,I'll try explain below...

    Your pistol won't get up to that power, it wouldn't,just because a gun shoots 410 fps with 1 pellet,doesn't mean it will with them all,pretty simple when you think about it.

    Airgun manufacturers often use alloy pellets that are super light to claim figured of 1000 fps etc.

    my HW40 runs at 2.1 ftlb,yours will do the same.

    Btw if your doing calculations regarding power,see if you know what's so special about 671 FPS.....
     
    Last edited: Jul 16, 2014
  17. Sir Oscar

    Sir Oscar Donator

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    Mr Hackers. After reading all your posts, not just this thread but the others you have started. It seems your intent on firing just about anything you can lay your hands on through your guns. Why not just use normal everyday pellets or *** and just enjoy. :) no need to push the limits of the law, it's still fun.
     
  18. jimmyhackers

    jimmyhackers Well-Known Member

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    i know 671 fps means that whatever grain my pellet is what power figure in ftlbs im shooting.

    i figured a bigger pellet means slower velocity......but again "assumption is the mother of all fookups" hence asking. and i did learn that pnuematic perform different to springers, and how.

    i dont use everyday pellets becuase as you may of figured....im not your everyday person.

    my intention isnt to break the law or push the limits. its to experiment primarily, a what if, a break from tin cans and holes in target paper.
    id just like to make informed descions and get prior advice from people who may have been there before.

    be thankful im asking questions because "stupid people dont ask question"
     
  19. darklord

    darklord Can’t beat a tx200 at 30m

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    The Police will use whatever the gun shop sells them primarily, My RFD and FEO told me that. The Bisley magnums is a myth certainly round here. its not not worth the time money and effort to prosecute someone with a 12.5ft/lb air rifle cause it goes over with a specific pellet that you don't use.

    The cost is stupidly high.

    Its different though if you actually do something daft such as shoot in public etc or they get a serious complaint, but they aren't interested otherwise. PS its the CPS who decide to prosecute not the police they only charge and evidence gather.
     
  20. darklord

    darklord Can’t beat a tx200 at 30m

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    Ive been shooting long enough to know i can't be bothered to explain now!
     

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