Will too much pressure cause extreme velocity spreads?

Discussion in 'Long Range Hunting & Shooting' started by supercrossbmx69, Oct 10, 2013.

  1. supercrossbmx69

    supercrossbmx69 Well-Known Member

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    So I just bought a 7 stw from a member of LRH and I'm working on getting a load ready for hunting season. He used h1000 with 168 Bergers but I was fortunate enough to find some 180 so that's what I'm using. Here's the problem, I started my loads pretty low and started working up with h1000 planning on it being my powder to use and didnt even consider trying anything else but retumbo. I only load one round at a time and move up in .5 increments. But for some reason the velocities are everywhere! For instance, I loaded up one at 77 gr and another at 77.5 the 77 gr load crony said it was flying at 3061 and the 77.5 was going 3039!!!! All my loads with h1000 are doing this no matter how low or high I go. But with retumbo, I loaded one at 75.8 which crony said 2987 76.5 was goin 2976 and 77.0 was going 3005?? So I loaded 10 at 76.5 and shot 3 through the crony and it went 3023 3033 and 3035?? Someone help please??
     
  2. Lefty7mmstw

    Lefty7mmstw Well-Known Member

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    Likely just a bit larger extreme spread than perfect combined with small powder charge changes and a small sample size. I jump up to a grain or two between stops for bigger cases and don't worry much about anything but the target to start with; I've got some loads I have been loading for better than a year that haven't been chrono'd yet but they shoot just fine in any condition.

    High pressure can cause big extreme variation numbers, especially when the bolt lets loose; pretty quick pressure drop off then and you'll have a big x for that group. Kidding on that one, sort of.:rolleyes: Much more of a safety and caring for your equipment issue, but your accuracy and uniformity can go to pot with too much pressure for sure.:D The easiest pressure measurement system I've seen yet is the shoe test; take off your shoe and tap the bolt open with it. If it doesn't go easily in a tap or two, you are likely over pressure.

    A 10 fps x with a 3 moa group is still a 3 moa group.
     

  3. Dr. Vette

    Dr. Vette Well-Known Member

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    I'd also wonder what chronograph you were using and the conditions under which you shot (sunny, cloudy, overcast).

    Do you wait the same time between each shot, or is it varied so that some might have a warm barrel while others do not?
     
  4. MontanaRifleman

    MontanaRifleman Well-Known Member

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    It could be your chrono... how far is it from the muzzle? Are the light conditions changing?

    What about your brass? Virgin? Sized?
     
  5. supercrossbmx69

    supercrossbmx69 Well-Known Member

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    Sun was not very bright when I shot through the crony but it was bright enough brass was twice fired and sized. Directions said be atleast 18 ft from muzzle for muzzle blast so I went to 20yds to be certain that muzzle blast wasn't the problem the crony I used is a beta crony with printer that you can get from sportsmans and it was very speedy!