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How do I get my extreme deviation or spread DOWN?

 
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  #8  
Old 08-21-2013, 07:27 AM
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Re: How do I get my extreme deviation or spread DOWN?

One other thing that will add to the muzzle velocity spread is how consistant the shooter holds the rifle against his shoulder. Folks who've compared muzzle velocities with a shoulder fired rifle and a machine rested one see a greater spread when the rifle's hand held. Us humans don't hold rifles against us exactly the same from shot to shot.
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Old 08-21-2013, 07:10 PM
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Re: How do I get my extreme deviation or spread DOWN?

I agree with folks about the chronograph issues. It wasn't until I went to 20' screen spacing with an Oehler, that I could see ES really well.
Other things
Weighed charges of course
Load development/components/seating depth/Load density
Consistent neck tension
Hold, as Bart mentioned
Consistent sizing as measured
Consistent fouling(oil burnt out/removed) and shot timing/bore temps
Matched case H20 capacities(as properly measured)
Firing pin/spring/trigger/action timing settings
Reduced chamber end clearances(trim length)
Reduced neck and throat clearances
Consistent shell loading with low load density loads
Lower muzzle pressures(long enough barrel for load)
Faster powders + higher pressures + good load density + low muzzle pressures (ala underbore BR cartridges)
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  #10  
Old 08-21-2013, 08:03 PM
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Re: How do I get my extreme deviation or spread DOWN?

Not to hijack the thread, but what is considered to be a low value for the deviation?
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  #11  
Old 08-21-2013, 08:18 PM
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Re: How do I get my extreme deviation or spread DOWN?

Regarding shot timing and bore temperature . . .

Shoot 3 or 4 rounds to get the barrel fouled and warm.

Fire each shot at the same time interval. 30 seconds, 1, 2 or 3 minutes apart. Whatever you want.

Chamber each round 10 or 15 seconds then fire it. Each round needs the same chamber time else its powder will be at a different temperature for each shot; velocity will vary with powder temp. Hotter powder produces faster velocities.

And use the "SAAMI Twist" which is a repeatable way of taking a cartridge from its box, turning it end over end, then holding it horizontal as its loaded in the rifle. Then the rifle is handled the same way each time a round's fired. All this does is repeatably distribute the powder in the case for more uniform ignition and burning. Page 95 and 96 in the following SAAMI link has the instructions.

http://www.saami.org/specifications_...wnload/206.pdf

Great values of standard deviation of muzzle velocity is under 10 fps. That said, many a rifle will shoot well under 1 MOA at 1000 yards with standard deviations in the 20's. That's caused by how the barrel whips in the vertical axis when shot. Slower bullets leave when the muzzle axis points at a higher angle while its swinging up; faster ones leave sooner while that axis is at a lower angle on its up swing. This often happens and it sometimes makes long range accuracy better than short range accuracy.
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Old 08-21-2013, 09:52 PM
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Re: How do I get my extreme deviation or spread DOWN?

Quote:
Originally Posted by jkupper View Post
Not to hijack the thread, but what is considered to be a low value for the deviation?
It really depends on what your range requirement and target size is. For example my 6.5x284 at 1000 yards, 2964FOS with a Berger 140VLD with an ES of 20FPS will calculate to about 4" vertical variability induced by this ES. At an ES of 10FPS it's 2". This can be calculated using a ballistic calculator by plugging in the velocity spread. While I generally try to get my ES around 10FPS for long range work for medium size game out to 1000 yards, 10-20 FPS will do. My current 6.5x284 load has an ES of 11FPS. The best ES out of my hunting rifles is 8FPS with my 300WM.
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  #13  
Old 08-21-2013, 09:55 PM
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Re: How do I get my extreme deviation or spread DOWN?

Quote:
Originally Posted by Lazerus View Post
Thanks for al lthe responses on my last post. I have a newly built 6.5 284 Norma built on a tika action with a 28 inch Burx barrel giving me some nice little 1/4 clusters at 100 yards. I have tried two powders, reloader 22 and H4831SC. Everybody says to go with Hodgeson so that is what I am going to do. So here is what my velocities are now:
2980
2957
2980
2985
2963
2981
2964
Basically I only hae a 100 yard range to work with and I only have CCI bench rest primers. I am shooting Berger 140 grain VLD's at 10.000ths down from full jam. I could probably move the bullet in alittle bit, but the group starts to open up if I move it out. I am just not sure what the practices are to get this extreme spread down. ANy advice would be greatly helpful...forgot to mention I am loading 52.8 grains of Hodgeson.
Put 2 off the shelf generic chronys back to back some day and you will never trust one again, not to mention what light conditions do to readings.

If you want accurate velocities, get a magnetospeed. Otherwise trust your groups down range. A tight ES doesn't necessarily mean a precision load. Last, SD means next to nothing unless you have large sample sizes.
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  #14  
Old 08-21-2013, 10:19 PM
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Re: How do I get my extreme deviation or spread DOWN?

Quote:
Originally Posted by Greyfox View Post
It really depends on what your range requirement and target size is. For example my 6.5x284 at 1000 yards, 2964FOS with a Berger 140VLD with an ES of 20FPS will calculate to about 4" vertical variability induced by this ES. At an ES of 10FPS it's 2". This can be calculated using a ballistic calculator by plugging in the velocity spread. While I generally try to get my ES around 10FPS for long range work for medium size game out to 1000 yards, 10-20 FPS will do. My current 6.5x284 load has an ES of 11FPS. The best ES out of my hunting rifles is 8FPS with my 300WM.
Thanks for the answer. I am currently getting an 18 fps variance with my 7 RM, and I though that was pretty good. I think I can clean it up a bit more, but it works for what I am currently doing. This thread really helps!
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