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Old March 8, 2012, 11:52 PM   #126
dacaur
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Quote:
Many benchrest shooters, in particular, have for years reported best accuracy just short of maximum in their gun,
+1 My best load turned out to be 43.8gr of varget, and at 44.2 gr it bucked me and the bolt was tough to open....
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Old April 6, 2012, 01:28 PM   #127
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OCW Development Progress for .308




reshot the last 3 charges same day



was suggested to try 46.2 as OCW and work on seating depth



2 shots in top of fouler targer were sighters and the lower shot belongs to the first group on bottom row. Not sure what the flyer in last group was but maybe I didn't let barrel cool off enough between shots
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Old April 6, 2012, 01:52 PM   #128
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hmmm, well, looking at those targets and some of your previous, I would have focused on 45.7-46... what made you pick 46.2?
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Old April 6, 2012, 03:07 PM   #129
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another person suggested 46-46.2 range based on my last targets??
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Old April 7, 2012, 04:38 PM   #130
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I think there's maybe some statistical faking out going on. From tables in the Lyman #46, a 3-shot group's diameter predicts that 95% of all future groups will be between 0.4 times and 2.5 times that same diameter as your sample group, and 5% will be smaller or larger than those limits. When I look at the image you put in post #121, it doesn't appear to me any of those groups falls outside those kinds of limits. That's why we went through the exercise of overlaying on tracing paper, to get 9 shot groups, which showed 46.5 grains actually to be the best center charge. Another step you might make is to switch to 5-shot groups which reduces the confidence diameters by 60%. With 5 shot groups, assuming nothing changes in the gun, you can figure 95% of future 5-shot groups will be within 0.67 to 1.5 times the size of your sample group.

The issue of the previously loose bedding is still on my mind. If the gun can shift in that stock at all, it may change the tuning, so check for consistency.

It is important to keep in mind that pressure and barrel time will both change as you tune seating depth. It is unlikely, therefore, that a single load will be the best at all seating depths you try, though it should give you some hint. If you got closer to the lands than you were when you made the groups in post 121, then you may have raised the pressure up enough to exceed the best accuracy as indicated by the tracing paper. Since this isn't a self-loading rifle, I would consider retuning by Berger's method, starting with the bullets jammed 0.010" into the lands. This will probably take about 10% off your current powder charges. Once you have that load, pressures will only go down as you seat deeper for the first 1/4 inch or so, with the result you should be lowering rather than raising pressure at the deeper seating depths.

Once you have a good candidate for best seating depth, you then need to retune the charge for it. I know of no way around that. I'm currently conducting an experiment seeking a better method of tuning seating depth, but don't yet have anything certain to report. I will when and if I get it to work.
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Old April 7, 2012, 06:07 PM   #131
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unclenick: 121 we were looking at was with RL 15 and this new targets are Varget
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Old April 8, 2012, 08:37 AM   #132
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Ah. Missed the changeup since I've been following our PM's but not the thread. Would advise the tracing paper again, though. No other easy way to see whether the net center stays in the same place over several charge changes. The upper orange target does look like it has 46.0-46.4 at about the same center, but the next target has 46.4 higher, making it appear the center charge may be at 46.0 instead of 46.2. 45.6 and 45.8 seem to impact a lot lower, so I wondered if you'd either changed point of aim or used a different sight setting with respect to the center.

Since the seating depth targets change in 0.040" steps instead of the 0.030" steps Berger recommends, I can't guess whether they skipped over a seating depth sweet spot or not. Berger's done careful experimentation on that, so I would not have assumed that 0.010" greater was going to work without proving it. That said, the 2.199 is clearly best among the seating depth targets you have, and at this point I would retune the powder charge there (though with only a couple of 0.3 grain steps either side of center to see if there's any improvement. If so, I would switch to that charge and then tune the seating depth in and out a couple hundredths to see if the center of the seating depth sweet spot can be better identified.
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