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Old August 6, 2012, 06:27 PM   #1
1stmar
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Acceptable case weight variation?

When you guys weigh cases what do you deem as acceptable for variation? Please use percentage as every cartridge case has different weights.

Also, does variation in bullet tension/pull typically show up through vertical stringing and velocity variation?
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Old August 6, 2012, 06:41 PM   #2
Creeper
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Quote:
When you guys weigh cases what do you deem as acceptable for variation? Please use percentage as every cartridge case has different weights.
I use a bell curve. Dependent on your intended use, from benchrest to general purpose plinking, and everything in between... you will have light, heavy and those that fall in between. Cull the lights and heavies and use the ones in the middle. The acceptable range is again, up to the intended use.

Quote:
Also, does variation in bullet tension/pull typically show up through vertical stringing and velocity variation?
Yes. Consistent case neck wall thickness, thru the use of a hand held neck turning lathe along with the use of bushing dies are the best way to obtain consistent neck tension and repeatable neck expansion.
For a standard, production chamber... determine the thinnest case neck wall thickness you will be using, and turn all cases to that value +/- .0005".

Don't over do it if you have a chamber on the large side of a production spec... as you will needlessly stretch the neck. Should the difference in an unturned loaded round and the estimated chamber neck dimension be greater that .006", I recommend not turning the necks.
Instead, try to find a brand of case with a thicker neck wall, which can be turned and still maintain a reasonable maximum clearance, or a case brand with no more than .0005" wall thickness difference.

For benchrest rifles with tight neck chambers... this is all information you would already know, and be putting to use.

I can provide additional information in greater detail, but as it's extensive, I'd provide it in a PM.

Cheers,
C
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Last edited by Creeper; August 6, 2012 at 06:46 PM.
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Old August 6, 2012, 06:59 PM   #3
1stmar
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Thanks creeper, I'm not competing. I won't get into neck turning, too much effort:-). I am culling the cases exactly as you indicated, had some very good results today virtually no horizontal dispersion across two loads (36.5&37 gr varget) and some minor vertical stringing, less the.4 and the velocity extremes seem to coincide with the stringing. Ss you would expect. So, i don't think powder variation is a concern, both groups over lap perfectly. My guess is neck tension, I could feel slight variations in tension during bullet seating. Trying to asses if it's worth scrutinizing my case weight more or just caulk it up to tension.

My Stdev is 25, extreme spread 59 avg velocity 3643 for 36.5 gr. 22-250
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Old August 6, 2012, 07:20 PM   #4
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You may be able to attribute your V and H spreads to wind and mirage... takes a long time to learn how to dope both successfully.

You can play with sorting case weights, measuring the neck wall thickness as a sorting factor, match primer pockets (not clean, but actually match them with a cutter), invest in a bushing die, get or make some wind flags, use a more stable and repeatable shooting platform... the list goes on forever.

I've shot bazillions of .22-250. It's a very addicting and fun cartridge to play with. Sooner or later... you'll get frustrated with your current limits of accuracy and stick your whole foot, as opposed to only your current big toe, into benchrest methods and practices. Cha Ching!

Cheers,
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Old August 6, 2012, 07:39 PM   #5
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I think that, for most of us, the difference in resulting case capacity is less than we would notice...

The wind, shooter error, neck tension (as you mentioned), barrel harmonics, all combine to make for a soup of variables to the extent that I think it comes down to trying to split hairs.

To know, you would have to be shooting under some really controlled conditions, where you have eliminated every other variable.

I recently bought some 6.5 Grendel brass for my son's AR- made by Lapua for AA.

I will admit I was amazed at the consistency of the weight of the cases. Rarely even a tenth of a grain. Were they any more "accurate" than the Hornady brass? Who knows... at 600 yards, who can tell why one POI is an inch apart from another?
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Old August 6, 2012, 07:45 PM   #6
1stmar
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Mirage is a strong possibility, first two shots were virtually in the same hole. Excited and impatient I didn't wait too long before completing my string. Additionally there was a lot of light change, clouds/sun etc..
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Old August 6, 2012, 07:58 PM   #7
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What magnification scope you using? The more mag, the more "boil" you can see. With some practice, you can calculate the change in POA/POI. No one is perfect at it, but it's too much of a variable to completely ignore.

When I shoot benchrest in AZ, or a state where it can get pretty hot, a 110 degree day and a 10 mph crosswind combined can put a 68 gr 6mm bullet as much as a 1" out of the group. It sucks when it's your last shot for record and the first 4 are in a .077" bug hole.

Try using a benchrest target, with a sighter target, a record target and a square aiming point. It's easier to divide a square with a cross-hair reticle than a circle. You know the dimensions, and if you need to hold a 1/2" or what ever for the wind and mirage, it's more repeatable.

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Old August 6, 2012, 08:10 PM   #8
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I'm using a b&l elite 6-24, on 24. I have some of those targets, I'll try them.
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Old August 6, 2012, 08:18 PM   #9
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Good scope with sufficient magnification.

Sometimes, matching a target to the distance, magnification, reticle design and subtention can improve quality of aim and cut group size considerably.
I've made loads of custom targets to match a unique combination... from old Colt 4X AR scopes, to 2 MOA red dots, to German #1 posts to 60X 1/8" dots.

Have fun with it, and don't wear out the barrel too fast.

C
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