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Old March 11, 2014, 08:39 PM   #26
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Quote:
Originally Posted by Bart
I once put 5 consecutive Sierra 155's from my .308 into less than 2 inches at 1000 yards while the cross wind speed was changing back and forth. Used aperture sights, not a scope. None of them went where I called them.
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Old March 11, 2014, 08:54 PM   #27
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Murphy resides at both of the extreme tail ends of bell-shaped statistical probability curves.
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Old March 11, 2014, 09:46 PM   #28
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This is an interesting thread, and one that should be read by a lot of new or mid-level shooters.
As an ex-academic that taught graduate level spatial probability and statistics years ago it always bugged me that people associate accuracy with the size of their groups. The size of of a group, or mean radius or whatever is the populate reference is really a function of repeatability which is a function of variance. But it's not accuracy.
That is, you could fire a very tight group (one with only small variance between each shot landed) but they all could be wide of the intended mark, or the target center.
Is that accurate? No, not really. Said another way, if you consistently missed your target placing shot after shot in the same hole it would be true that you are consistent, low variance, extremely repeatable. But you'd not be very accurate.
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Old March 12, 2014, 03:39 AM   #29
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If a very tight group was a little wide of the bullseye, a few clicks on the horizontal adjustment of my Weatherby's scope would bring 'em right in there .... every time.
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Old March 12, 2014, 08:20 AM   #30
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hooligan, that .0077" record's villafied in:

http://bulletin.accurateshooter.com/...th-0077-group/

Regarding my "lucky 5" at 1K, such is the statistical norm when all the variables cancel each other out. Had they all added up in different directions those 5 shots would have gone into about 20 inches. I called them inside about 10 inches on paper but the wind changed after I made a correction from observations in my spotting scope to when those shots were fired some 10 to 15 seconds later. My scorekeeper acknowledged that after that 15-shot string was finished as he also was watching the wind change back and forth while I was on the gun. His comment after that string of fire was "Welcome to Las Vegas. What name should we give your slot machine shaped like a rifle held by a human?" We both laughed.

It's my opinion that the smallest groups are happenstance when all the variables cancel each other out to the sum of zero. Or everything is perfect without any variables; the sum of all zeros in any direction is zero. How does one prove which one it was?
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Old March 12, 2014, 07:18 PM   #31
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It would be interesting to witness a calibration of the instrument that measured that .0077" group.

Last edited by curmudgeon1; March 12, 2014 at 11:28 PM.
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Old March 13, 2014, 09:55 AM   #32
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curmudgeon1, calibration of the precision calipers benchresters use to measure their groups is easy. I've calibrated a couple of my own in a metrology lab on Jo blocks and noted any error. It's easy to measure groups that small as their outside edges are "blackened" quite accurately in the special tagboard paper used for targets; a single bullet hole diameter's blackened edge is exactly what the bullet is; within .0001". I've measured bullet holes made in that paper with bullets .3075" diameter (groove diameter of the barrel they came from) with a 20X loupe and their blackened edge is just that. Benchresters often use single bullet holes on sighter targets to "calibrate" their calipers for the group holes in the target.

Resolution is easy with magnifying glasses observing the edge of the caliper's caliber ring to less than .0001" As many precision machinists with high levels of skills and knowledge measuring tiny things play that game, they've no issues with scorers in stool shooting matches well qualified to meaure precicely to resolve dimensions to that level. And the average of several measurements ends up being close enough to exact and actual for benchresters' objectives.
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Last edited by Bart B.; March 13, 2014 at 10:12 AM.
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Old March 13, 2014, 09:08 PM   #33
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Bart B., most experienced shooters have no problem using digital calipers to measure distribution of a shot group, but it may be a slight stretch for someone to claim a "new record" .0077in. over the old .0090in. (.00065) per side) record using eyesight alignment of the caliper edges to the ragged/blackened edges of the bullet holes, don't you think ?
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Old March 14, 2014, 12:38 AM   #34
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This has turned into one heck of a read guys! Thanks for sharing such knowledge.

It's funny how you go about learning things as you strive to perform better at certain tasks. When such a wealth of information is dropped in your lap, you realize how much farther you have to go even though you feel as if you've achieved so much.
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Old March 14, 2014, 09:40 AM   #35
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curmudgeon1, I believe that with a decent loupe on the calipers placed over the gauging rings on its glass plate, one can easily discriminate within a couple ten-thousandths where a blackened edge is relative to the gauging ring. A 10X loupe makes a .001" gap look like a .010" gap to the naked eye.

Here's a link to a popular one used these days:

http://bulletin.accurateshooter.com/...il-jones-tool/

You can see light coming through caliper jaws when they're only .0002" apart.

It does help to have exceptional vision.
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Old March 14, 2014, 03:27 PM   #36
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Okay, your knowledge and experience has now convinced me that the equipment shown can accurately measure bullet-hole dispersion in .0001's".
Kudos to the new .0077" champ. Now, no doubt these benchrest-shooters have given this one some thought: when someone eventually gets five bullets through the same hole, how will they know ?
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Old March 15, 2014, 04:00 PM   #37
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There's a roll of paper moving behind the stationary target they shoot at. With a string of shots clustered in one hole barely larger than bullet diameter, there are separate ones on that backer target.
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Last edited by Bart B.; March 15, 2014 at 04:11 PM.
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Old March 15, 2014, 05:40 PM   #38
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Ingenious ! Somehow I knew your experiences would have met up with that situation along the way. Some guys will be interested in your answer. Thanks.
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