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? about point of impact with different powders

hunter86004
I've got something strange (for me anyway) about the point of impact on two powders I just loaded with.
I loaded up some 180 grain spire points for my '06 and used 50 grains of IMR4350 for some and 49.5 grains of IMR4895 for another batch, both out of the same box of bullets. Both delivered tight 3 shot groups at 100 yds, but the groups were about 2 inches apart horizontally. Fired another 3 shot group with both and had the same result. I had expected they might group different, but vertically due to different speed, but horizontally?
Anybody got an idea as to the cause?
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Comments

15 comments

Date Votes
  • clownboy
    I am probably not qualified to answer this as I have my own accuracy problem. But I have been reloading for 12 years and have an idea what may be going on. Different powders burn at different rates when ignited. The rate also changes the velocity of the bullet which in turn will cause it to raise or fall and even fly differently. This is what I believe you are experiencing.

    Hope others chime in to either confirm or denounce my theory.

    Brad
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  • wtroper
    I also make no claim to be an expert on this subject. However, I too am surprised that .5 gr of powder move the point of impact by 2" at 100 yds. However, if it did, it did.

    My layman's opinion is that it has to do with the harmonics of the bbl. The barrel was in a different position when the bullet exited. I presume that your rifle is not full length glass bedded.

    Good info to have. Must be careful in weighing the powder.
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  • Rocky Raab
    He used TWO kinds of powder, wtroper. I think you missed that.

    That's not at all uncommon. Different powders will develop different pressure/time curves and thus will also produce different bullet flight, even if the velocity is nearly the same.
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  • hunter86004
    I just find it rather strange that the two loads would give a pair of horizontal rather than vertical groups.
    And no, it's not grass bedded...just a Ruger 77 out of the box.
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  • hunter86004
    Make that GLASS not GRASS. LOL
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  • dcs shooters
    Two different powders with different burn rates=different barrel harmonics. Glass bedding and free floating might help.
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  • hunter86004
    I'm quite happy with the group size and it's only used for elk and letting the grandkids run cast bullets through. Just curious.
    Thanks all for the input.
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  • BGHillbilly
    It would almost have to be barrel harmonics, different pressures/velocities and the muzzle is pointing ever so slightly different with the different powders when the bullet exits. Sort of the reason you find a good combination powder and bullet and stick with it for hunting. Pick one of the powders and let the grandkids shoot the other along with thier cast bullets. Just let debris of some kind such as a leaf or blade of grass get between the barrel and stock and see what happens, it changes the harmonics with every shot.
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  • jonk
    Barrel harmonics, velocity, spin and stabilization rate all have an impact.

    FWIW 49.5gr of 4895 and a 180 gr bullet sounds quite hot to me.
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  • haroldchrismeyer
    quote:FWIW 49.5gr of 4895 and a 180 gr bullet sounds quite hot to me.

    I agree, since my books show 47 being max. But knowing 4895 quite well, I don't think you can get enough of it in the case of any cartridge to blow something up.

    When you push maximum in anything, strange things happen. I agree with barrel harmonics too, especially when using hot loads.
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  • hunter86004
    RE: FWIW 49.5gr of 4895 and a 180 gr bullet sounds quite hot to me.
    My Lyman book shows a max of 47.5, but my Hornady shows a max of 50.3.
    It is a bit warm, but I haven't noticed any primer deformation, nor have I experienced any problem with extraction.
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  • ddhotbot
    http://optimalchargeweight.embarqspace.com/#

    take a look at this link it will explain everything about harmonics
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  • guntech59
    quote:Originally posted by ddhotbot
    http://optimalchargeweight.embarqspace.com/#

    take a look at this link it will explain everything about harmonics


    Excellent site. I will try that when working up my 100gr load for the .243.
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  • ddhotbot
    http://practicalrifler.6.forumer.com/

    this is a forum started by the same guy .
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  • Mtnlover
    I once ran out of very old (from my Dad) IMR4350 for my first centerfire, a .243 Win M70 Featherweight and bought a new can of the same powder. The groups, still very good, shifted about 3" to the NW. I'm betting a slightly different burn rate changes the barrel harmonics, especially in a very light barrel.
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