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flier caused by scope cant and accuracy evaluation

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Unread 07-19-2003, 10:20 AM
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Join Date: Jul 2001
Location: Walla Walla, WA
Posts: 174
flier caused by scope cant and accuracy evaluation

I am working up a load for my 7mm Rem Mag using the 160 Nosler Accubond. This is my backup big game rifle. I hunt on foot so no bench shooting at game. Very, very long range shot for me would be 500 yards (I did take a deer with this rifle at 508 yards – I was prone with a bipod- with a load using the Barnes 140XBT blue coat but then they changed the ogive shape and I have not been able to get it shooting good again). Anyway, I wonder how you all evaluate a loads’ accuracy. I generally look at a few criteria. First of all I want a low extreme spread in velocity, hopefully less than 50 fps with a standard deviation of less than 15. Secondly I want group size to average as small as possible but for big game I hope to average about 0.7" 3 shot groups at 100 yards (from the bench). Then I hope to have a consistent group center such that if I shot 3 or 4 groups at the same spot I would have 3 to 4 groups that averaged 0.7" but all 9 to 12 shots would be in about 1.25" or so. That is my goal but it is hard to achieve 9 to 12 shots in 1.25". I shot this evening at 400 yards with what I think is a suitable load with this bullet in this rifle (a pre-64 Winchester Model 70 with a newer Shilen barrel). First group was 2.8". Second group was 5.5" with one shot quite a bit lower and to the right of the other 5. 5 shots were in 2.8" at this point. Final group was 1.8" but its center was a bit higher and left of the 5 that clustered together from the first two groups. Overall 9 shots were in 9" at 400 yards with 8 of those 9 in 4.8" (1.2 MOA). Basically I think I am there with load development but I sure hated seeing that flier in the second group that was so low and right of the other 8 shots. The scope is a Leupold Vari X3 2.5-8. I do not have an anti-cant level on it. I wonder how much cant it would take to cause a shot to go 4" right and 4" low of the group center of the other 8 shots? Would that amount of cant be visibly noticeable at 400 yards at 8x? Do you guys look only at average 3 shot group size or do you look at what the group size would be if for example 4 separate groups were fired at the same spot? Rufous.
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