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Changing point of impact? Picture attached

Discussion in 'Rifle Country' started by greg788, Jan 19, 2009.

  1. greg788

    greg788 Active Member


    Is this normal from one 3-shot group to the next? I'm referring to the 3 groups at the bottom that I highlighted. These 3 are all the same load (Winchester Super-X 100 grain) at 100 yards off a benchrest with a TC Icon .243.

    The point of impact seems to be shifting slightly each time. What could be causing this?

    I can't complaint about group size, which is right around 1".

    Also, I get a lot of groups with 2 shots close to each other and then the third further away. I assume the first two shots are close to each other then the third gets thrown off like a minor flyer but I haven't checked for certain yet (don't own a spotting scope). What could be causing this issue?
  2. alpha6164

    alpha6164 Well-Known Member

    Does your scope have parallax adjustment? I am wondering if your cheek weld is slightly different every time?
  3. briansmithwins

    briansmithwins Well-Known Member

    3 shot groups are next to useless as chance plays too large of an effect.

    I suspect that if you superimpose all those 3 shot groups you'll see the true accuracy of your rifle, which is going to be larger than 1".

  4. franconialocal

    franconialocal Well-Known Member

    It's all about the breathing...............:cool:
  5. rangerruck

    rangerruck Well-Known Member

    Everything appears to be high and to the right. If you are a right handed shooter, i would say the problems go this way, from strongest to least.
    You; are pulling and anticipating the shot. the stock, is touching as it heats up, particularly somewhere on the left side of the bbl channel. You should heat it up fast, with about 20 shots, and test the freefloat with a dollar bill, out on the range. Scope; generally a bad scope will shotgun pattern the groups, these are a little too uniform for me for a bad scope, however, that does not include your rings may have a hairline crack in them. To check for this, put a wood dowel in them, while they are mounted and tightened down, and twist the dowel. see if a small crack opens up somewhere on the rings, look real close.
  6. briansmithwins

    briansmithwins Well-Known Member

  7. Walkalong

    Walkalong Moderator

    A very small change in wind will do that to you. Were you trying to shoot in the same conditions everytime, or just shooting.

    Basically all your "groups" are to the right, with fliers, either the wind, or you. I suspect gun handling. Shooting off a rest takes practice, just like all shooting.

    You could try a different scope, just to rule that out, or perhaps find the problem.

    MT GUNNY Well-Known Member


    How many rounds in a group do you think it takes for reasonable assessment?
  9. MCMXI

    MCMXI Well-Known Member

    greg788, it looks like the average centers of the bottom three groups are in the same spot on the target. If it were my rifle/scope, I'd adjust the reticle to move the POI 0.5MOA down and 0.5MOA left and then work on tightening up the groups.

    I think you need to shoot at least five shots per group and if you're working up loads, the groups should be shot using the "round robin" approach as shown below. I've just started loading a new bullet for my .300 Win Mag (a 208gr A-MAX) and shot the five targets below yesterday. The loads increased by 1.0 grain increments from 67.5gr to 71.5gr. I shoot prone with a bipod with NO bag or rest under the butt, just my shoulder. I'll admit that it takes me longer (and more rounds) to figure out the best load but I get a lot more PRACTICAL shooting practice that way. Five-shot groups provide more meaningful data than three-shot groups. The first target below is a six-shot group (I had an extra round from sighting in) measuring 0.63". There are single fliers on targets 3 (0.81") and 4 (0.69") but the POI is the same. If you discount the single fliers on those targets the groups shrink to 0.47" and 0.47". All of these groups were shot with brand new brass, used as is without trimming but they were sorted by weight and the bullet was seated 0.020" off the lands. I don't think there are fliers on targets 2 and 5 per se ... the load seems VERY unstable at 68.5gr and is starting to come apart at 71.5gr. If you shot these targets with three-shot groups, I doubt that you'd get anything like the data that I got from five-shot groups. One last comment, you might think (due to the group) that the 67.5gr load is the best. However, despite the fact that it grouped well, there are two problems with that load. First, it's going to be pressure sensitive since 1.0gr more powder and all hell breaks loose. My thought here is that it's the upper end of a vibration (frequency) node. Second, the velocity of that load is going to be as much as 200 fps slower than loads around 70.0gr. Based on these targets, next weekend I'll try 69.0gr, 69.5gr, 70.0gr, 70.5gr and 71.0gr to see where the lower and upper limits of the load are.



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