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Shooting Forum - Firearms licencing information


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POI change for different ammo

  • 09-06-2019 12:18pm
    #1
    Kran
    Registered Users Posts: 231 ✭✭


    Just zeroing my .223 and I’m using Hornady 55gr Fiji/BT and the box gives muzzle velocity of 3240. I use these just for zeroing a new scope but I hunt with Hornady V-Max 55gr which also says muzzle velocity of 3240. Shooting a Remington 700 and my groups with each are very good but changing between the two there is a few inches of difference on the horizontal plane. Is this to be expected? It happened me also when I first zeroed this rifle earlier in the year. It makes no real difference as I just zero with my hunting round and let it be but an explanation would be nice.


Comments

  • #2
    alanmc
    Registered Users Posts: 1,132 ✭✭✭alanmc


    In my limited experience, I'd say yes, this is expected behaviour. Different BCs and shapes of bullets affect POI even if they have the same muzzle velocity. How it moves through the air, how it sheds speed, etc.


  • #3
    cookimonster
    Registered Users Posts: 2,759 ✭✭✭cookimonster


    It can be as amusing as this-

    Two Remington 700's with exactly the same specifications firing the same ammo but one will shoot to a different POI then the other. There's too many anomalies to be counted that may make a difference.

    But in reality you are launching two different types of bullets and the external ballistics effecting the two will also be different. The grain weight of the bullet is consistant as is the calibre but the lenght of the bullet will / can differ. The shape of the tip and its ojive will have different aerodynamics as will the base of the bullet wether it is flat or boat tailed. Each factor can effect the trajectory / path of the bullet.


  • #4
    clivej
    Registered Users Posts: 7,027 ✭✭✭clivej


    Barrel harmonics


  • #5
    Zxthinger
    Registered Users Posts: 1,113 ✭✭✭Zxthinger


    clivej wrote: »
    Barrel harmonics

    There's more than that too. It's my opinion that certainly harmonic effect poi. But different bullets leave different fouling with means that you can't just swap and change like you like.


  • #6
    BryanL
    Registered Users Posts: 1,124 ✭✭✭BryanL


    Their aerodynamics are different, same as their terminal performance would be different too.
    Your comparing the same size apple to the same sized orange.


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  • #7
    Kran
    Registered Users Posts: 231 ✭✭Kran


    Thanks everyone, I was just double checking that it was nothing out of the ordinary.


  • #8
    Zxthinger
    Registered Users Posts: 1,113 ✭✭✭Zxthinger


    Different aerodynamics! Yes.
    Different velocity! Yes
    Different bullet weight possibly
    Different harmonic shock wave yes
    Different ballistic coefficient

    With that said if loads could be worked on then it might be possible to tune both such they the difference in POI at a set range is similar to the theoretical differences in the drop charts at that range. However I couldn't reload so I couldn't test that. What I found was that it took several rounds of any one ammunition type to establish consistent impacts. IOW I found that I couldn't just chop and change ammo types as I liked-


  • #9
    meathstevie
    Registered Users Posts: 4,772 ✭✭✭meathstevie


    Any amount of variables relating to the difference in ammo can easily cause those relatively slight shifts of POI. An inch or two at 100 metres for a completely different cartridge out of the same rifle is not that wildly a shift either.


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