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Thread: Different brand .22LR, different lateral POI: Why?

  1. #1

    Different brand .22LR, different lateral POI: Why?

    If I shoot with different brands of subsonic HP .22LR I'm not surprised that I get a different POI.

    I can easily understand why the vertical POI might vary, due to different BC, bullet weight, charge, etc, but why does the POI change laterally with some brands as well? I've found that the lateral difference can be as much as 1" at 55 yards. This happens even when the bullet weight and muzzle velocity are virtually the same between brands.

    Is it due to barrel harmonics and/or something else? And why? I'm just intrigued.

  2. #2
    As you have already stated.... Variables!!

    You also forgot to add Bullet diameter, Lube used, the effects of you chamber, torques settings.... etc...etc....

    This is why most shooters test small batches until they find something their rifle likes, then buy as much as they can of that "Lot" until it runs out.

  3. #3
    Quote Originally Posted by Ratwhiskers View Post
    As you have already stated.... Variables!!

    You also forgot to add Bullet diameter, Lube used, the effects of you chamber, torques settings.... etc...etc....

    This is why most shooters test small batches until they find something their rifle likes, then buy as much as they can of that "Lot" until it runs out.
    Yes, but WHY should variables like bullet diameter, lube used, etc make the POI shift laterally?

    I don't think torque settings are relevant to POI shift with different brands of ammo, unless you're suggesting that different brands need different torque on the action screws. Or am I missing something?

    It's awfully quiet here, so has no-one any actual explanation about this lateral shift in POI? Surely I can't have stumped you all?!

  4. #4
    Because your barrel is a corkscrew not a pipe
    when the bullet travels up it the barrel it wants to move laterally and vertically

    different ammo will leave the barrel at different points in that movement
    same reason heavier bullets can often have a POI higher at the same distance than faster lighter rounds out of the same barrel

    its about when the bullet leaves the muzzle not the trajectory

  5. #5
    Just have a look at this bit of video of an airgun pellet in flight - you can often see the same thing when shooting subs in the .22.

  6. #6
    Quote Originally Posted by Humpers View Post
    If I shoot with different brands of subsonic HP .22LR I'm not surprised that I get a different POI.

    I can easily understand why the vertical POI might vary, due to different BC, bullet weight, charge, etc, but why does the POI change laterally with some brands as well? I've found that the lateral difference can be as much as 1" at 55 yards. This happens even when the bullet weight and muzzle velocity are virtually the same between brands.

    Is it due to barrel harmonics and/or something else? And why? I'm just intrigued.
    Virtually the same, and THE SAME, aren't the same thing. F' rinstance; A different nose profile can induce gyroscopic precession, causing the lateral shift. Also, two brands of ammo may get identical velocities but not operate at the same pressures.

    You just can't take any of the little details for granted, expecting two different brands to act the same be cause they are similar. My guesses? a A difference in velocity, or B) Precession as mentioned above.~Muir

  7. #7
    Thank you all for your thoughts.

    Quote Originally Posted by bewsher500 View Post
    Because your barrel is a corkscrew not a pipe
    when the bullet travels up it the barrel it wants to move laterally and vertically

    different ammo will leave the barrel at different points in that movement
    same reason heavier bullets can often have a POI higher at the same distance than faster lighter rounds out of the same barrel

    its about when the bullet leaves the muzzle not the trajectory
    Wow, the idea that my barrel is a corkscrew is going to take some getting used to. Is it that the barrel itself is actually corkscrewing while the bullet travels along it, or is it the bullet precessing within the barrel, e.g. due to the lands biting in more on one side of the bullet than the other?

    Whatever the reason it sounds like a great excuse for whenever I shoot badly; "yeh, my barrel is a corkscrew y' know!".

    Quote Originally Posted by Muir View Post
    Virtually the same, and THE SAME, aren't the same thing. F' rinstance; A different nose profile can induce gyroscopic precession, causing the lateral shift. Also, two brands of ammo may get identical velocities but not operate at the same pressures.

    You just can't take any of the little details for granted, expecting two different brands to act the same be cause they are similar. My guesses? a A difference in velocity, or B) Precession as mentioned above.~Muir
    Don't worry, I wasn't expecting different brands to behave in the same way, just interested in why apparently very small differences in ammo can make such a big difference to lateral as well as vertical POI.

    In case anyone else is interested (probably not!) I found this website, which although it refers to supersonic ballistics it explains some of the physics which is also relevant to subsonics. Just glad I stick with subsonics so I don't get into all the transonic malarkey.

  8. #8
    Quote Originally Posted by Humpers View Post
    Thank you all for your thoughts.



    Wow, the idea that my barrel is a corkscrew is going to take some getting used to. Is it that the barrel itself is actually corkscrewing while the bullet travels along it, or is it the bullet precessing within the barrel, e.g. due to the lands biting in more on one side of the bullet than the other?

    Whatever the reason it sounds like a great excuse for whenever I shoot badly; "yeh, my barrel is a corkscrew y' know!".
    like a load of jelly!






  9. #9
    Quote Originally Posted by Muir View Post
    Virtually the same, and THE SAME, aren't the same thing. F' rinstance; A different nose profile can induce gyroscopic precession, causing the lateral shift. Also, two brands of ammo may get identical velocities but not operate at the same pressures.

    You just can't take any of the little details for granted, expecting two different brands to act the same be cause they are similar. My guesses? a A difference in velocity, or B) Precession as mentioned above.~Muir
    The other thing is the thickness of the rim. Only discovered this a while ago. Because the manufacturing process is so fast some rims get compressed less , you can buy a rim thickness gauge and sort you rounds into groups , this is a really handy way of sorting bulk ammo into plinking ammo and target ammo . Heres a vid that explains the idea , love subs meself

  10. #10
    Quote Originally Posted by Sako jako View Post
    The other thing is the thickness of the rim. Only discovered this a while ago. Because the manufacturing process is so fast some rims get compressed less , you can buy a rim thickness gauge and sort you rounds into groups , this is a really handy way of sorting bulk ammo into plinking ammo and target ammo . Heres a vid that explains the idea , love subs meself
    Thanks for that video, as I know several people have suggested measuring rim thickness to improve accuracy but when I've asked no-one yet has actually confirmed to me how much difference it actually makes to group sizes. This video shows it, albeit for only one group.

    Do you use the Hornady rim thickness gauge or something else, e.g. I've heard that an empty .223 or .240 cartridge case can be used to "seat" the .22LR cartridge in and measure the rim thickness?

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