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Thread: Murphy, a little info please

  1. #1
    Member Adventures's Avatar
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    Default Murphy, a little info please

    My gun range set up I have has 2 55 gallon drums with chicken wire on the inside around the edges fastened end to end (this is to muffle the sound as to not anger the neighbors) The only problem I have is the chronograph is in the second barrel at the end so I'm about 6 feet or so from the muzzle. Any ideas how much velocity i'm losing in 6' with the .338 ultra. I'm getting really varied results with the chronograph sometimes the rounds will be up over 3150 and sometimes right around 3000 with the same powder. I've tried RL22 which was very accurate (all touching at 100 yards. and I tried 4350 not as accurate towards the top of the load range but muzzle velocity was a little better. Obviously i'll go with the better accuracy rather than picking up 50-100 feet per second. I'm going to continue tinkering with the powder and see if i can find one that works even better. All rounds are 225 gr partitions. I modified the clip so that I could get the bullet up to .027 from the rifling without taking the entire front of the clip out.
    Shot a 6' blackie with it last week. Overkill I know, but i was dying to use my new toy.

    Thanks in advance,
    Justin

  2. #2
    New member George's Avatar
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    Default muzzle vel reading

    UHHH not Murphy, sorry, and no info on loads but... 6' from muzzle is way closer than standard for taking "muzzle velocity" readings. Did I read that right? To my knowledge most of the MV data passed around and/or published is taken at something like 15', more or less. I don't think velocity loss at 6' from muzzle is going to matter one way or another. Plus, the debris cloud containing gas, smoke and remnants of the powder charge may not have had enough time to separate itself from the bullet at 6' so you may be getting erroneous readings anyway??

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    Default Chronographing and other technical stuff...

    George is right. Get the screens out a ways. I would not have much faith in readings six feet away. The standard is fifteen feet from the muzzle, five yards. Most programs, and online calculators, will give velocity with the input of B.C. and muzzle velocity in five yard increments. Just adjust it your data to read your instrumental velocity at five yards and it will give you a very close approximation of muzzle velocity. Also, fire five rounds and take the talley for a more accurate calculation. The velocity that the chronograph reads is called instrumental velocity, it is then corrected to the muzzle velocity by several methods, the easiest of which is with the Ballistic Programs. I would say that your 3100 fps Ultra is loosing 60 fps in six feet.

    I fire five rounds and do a summation on the Oehler model 33P which gives highest, lowest, extreme spread, mean (not average), and standard deviation. All useful date. I have two of these and can put one at 100 yards (be sure of the shot) and one at 15 feet and can calculate a pretty good B.C. for the bullet.

    When shooting so close to the screens the muzzle blast will cause the screens to wobble and induce an error. This is very common and even seen with big calibers at fifteen feet. Also, the unburned powder can trigger the sensors and cause erroneous readings. In the 338 Ultra I would expect to see no worse than about 60 fps from fastest shot to the slowest (Extreme spread) and a standard deviation (SD) of about 25-30 fps. These big cases are not very uniform in velocity. Good shootin'.

    Murphy

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    Member Adventures's Avatar
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    Wink Good to know

    Until we designed this sound depressor we always had the chronograph right up next to the gun. I had no idea about the 15 ft thing. Perhaps that's why my reading have so much variation. I'll give it a shot. I've been shooting 4 rounds with 10 mins in between rounds for barrel cooling. I've also been cleaning the rifle in between the 4 rounds of powder before I change to the next grain size (something my gunsmith recommended).

    Thanks again for the info guys.
    Justin

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