Standard deviation

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  • Last Post 18 April 2008
Lefty posted this 16 April 2008

If this question has been discussed before I apologize.  Yesterday I chronograghed three potential loads for my production rifle.  These are are fairly standard loads that appear here and in the FS.  The standard deviations for these loads ranged from 10 fps to 33 fps.

I have read that while a low std dev does not guarantee a good load, it is necessary, to have a good load.  Coincidently I had been using the high std dev load for the first two postal matches and the results had not been acceptable.  How important is std dev in developing a competitive load.

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PETE posted this 16 April 2008

 Lefty,

  What you've heard about SD is correct. A low SD is no guarantee of the most accurate load. But a load that doesn't have a reasonable SD won't be the best either. I try to find a load that will shoot to at least 10 fps. For BP I look for those around 5 fps. I had a 5 shot group once that went 0 fps. And many that went 2 - 3 fps. This wasn't the norm and is why you have to average what you get from several groups..... at least.

  The lowest SD with the best accuracy is what you want but seldom achieve.

  The reason why a low SD, or ES doesn't always work the best is due to barrel vibrations. If the bullet doesn't exit the muzzle at the same point in the vibration cycle every time a 0 SD won't shoot. From this you can also see tho that a low SD is really necessary but not the only thing that needs to be taken into consideration.

  You have to be very careful in how you interpret the info you get from your chronograph. A chrono is just a tool you use to narrow the search down for that best load. In the end it's what the accuracy is at the range you want to use that load for that really counts. I shoot very accurate loads where the SD is in the 20's, but nothing lower will shoot as well. I have loads that shoot like a house afire at 100 yds. but aren't worth a darn at 200.

PETE

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billwnr posted this 16 April 2008

Lefty,  how do these loads perform on paper?  Which one has the least “flyers” or deviations from the standard group?

 

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Lefty posted this 16 April 2008

billwnr

45 mph winds yesterday precluded any accuracy work. 29 gr of 4895 was the load with the 33 fps std dev.  Through 300 rounds over the last month, it looked like 1 1/4 to 1 3/4 were going to be the average groups.  Every 3 to 4 groups a flier would open things up to nearly 3 inches.  Shooting for score a typical target would be three 10s, one 9 and a 6.  I have tried 3 or 4 different bullets and the pattern has been basically unchanged.  This load obviously works well for some but I am ready to give up.  These are bumped bullets in a throated rifle so bullet fit  may not be perfect but it is not bad.

The other two loads, 18.5 of 4759  and 16 of 2400 both had std dev of 10.  These were measured with one 10 shot group each so could vary with more extensive testing.  While both of these loads have shot well for me and others in the past, I have not given them a good test in the current rifle.  Tomorrow could be the day if the wind will finally subside some.

Lefty

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billwnr posted this 16 April 2008

Does the odd shot show an odd velocity?  Is there a relationship?   How much lube are you putting on the bullets?   How often to you clean your barrel and what is your method of doing so?  Do you follow the same cleaning routine every time you clean and are you shooting the same number of shots between cleaning applications?

Inquiring minds would like to know.....:coffee

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Lefty posted this 16 April 2008

billwnr

I don't know if the odd shot shows an odd velocity as I have not chronoed an entire range session.  Not a bad idea though.  I clean the barrel every 12 to 20 rounds deopending on the number of sighters.  I use Eds Red and the same routine:2 wet, 10 brush strokes, 2 wet, 1 dry, one or two foulers.  the only thing unusual in the lubing is after normal lubing (white label x-lox) I lightly tumble lube in Lee liquid Alox  to reduce friction during the bumping process.  In 15 years at this game I have never had trouble with too much lube but I have never used the tumble lube before.  In a previous post I inquired if liquid Alox  used in this manner caused unusual fouling.  The first wet patch is met with some resistance.  All is smoooth after that.  Any ideas are appreciated.

Lefty

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billwnr posted this 16 April 2008

Two more questions for you. 

Do you inspect the first patch for the amount of fouling it pushes through and also for lead flakes?

Do you use a brass brush in your cleaning?

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Lefty posted this 16 April 2008

no unusual fouling on the first patch - just a little resistance as I push it through.

no leading - usually not a single minute flake

brass brush

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CB posted this 16 April 2008

billwnr wrote: Does the odd shot show an odd velocity?  Is there a relationship?  

I record each shot and log an out shot in my head to see if it went out. Usually if I get a high or low velocity reading, the shot still goes right in the group! :P  I'm not concerned about SDs and ESs. I find most good loads will stay around 35-40 ES or better.

It doesn't matter how I clean, as long as the barrel is clean. Sometimes I only wet patch with Ed's Red and sometimes I deep clean with a brush and Blue Wonder, at least once a match and after each match.

Lefty, when you find a good load with a certain bullet, log in the velocity. With another powder close to the same rate, your accuracy load for it should still be a close to that same velocity. Example: AL2400 shoots good at 1,620fps, SR4759 probably will shoot good also at 1,620fps. H4895 at 1,865fps shoots good, Varget will probably shoot good also at 1,865fps. This isn't a positive thing, but I bet it'll be close most of the time.

Powders are powders, sweet spots are sweet spots. The powders just get you into the spot. :cba:     ............Dan

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linoww posted this 16 April 2008

I shoot very accurate loads where the SD is in the 20's, but nothing lower will shoot as well. I have loads that shoot like a house afire at 100 yds. but aren't worth a darn at 200. Pete

I tested two very accurate small bore guns one whole wekend to pick out a lot of ammo for the year.One was a Remington 40X the other a Anschutz 2013.All testing was 5 shot groups at 100 yards.Some groups were as small as 1/2” in the calm morning conditions.My partner could call high or low shots just looking at the chronograph and 90%+ of the time the low SD's resulted in smaller groups.Wolf Target was the only one that had bad uniformity,but still shot well and it still baffles me.When we moved to 50 yds it was a bit harder to see the difference in group sizes to be able to pick one over the other.I was glad my experianced friend talked me into using 100 to sort my ammo choice.Being cheap I ended up shooting the Wolf ammo<G>

George 

"if it was easy we'd let women do it" don't tell my wife I said that!

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runfiverun posted this 17 April 2008

lefty you didn't mention what powder you were using

but you may have powder position issues, you know level,level,level,front

or primer consistency issues?

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Lefty posted this 18 April 2008

My reply above on April 16 - 10:00 AM  has the loads.  I treated all loads the same as far as powder position.  Transport the bullets base down - load carefully never allowing the bullet below level-close the bolt slowly.  They should have been basically level but some type of toilet paper wad will be one of the things I test soon.

I am shooting a postal match with the 4759 load today.  That should tell me something about fliers (or not).

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Lefty posted this 18 April 2008

I shot a postal match this morning.  The groups portion of that match consists of 4 - 5 shot groups.  I had shot two postal matches previously with another powder.  I don't know if the results you see below are a function of velocity, standard deviation or something else,  The load, the gun, the range and even the bench are unchanged.  The powder is the only change.

1st 2 matches  powder = 29 gr H4895 velocity = 1875 std dev = 33 fps, Agg for 8 - 5 shot groups = 1.60 inches

todays match  powder = 18.6 gr 4759  velocity = 1675 std dev =10 fps,  Agg for 4 - 5 shot groups = .88 inches  The score target today was a personal best with no shots outside of the 9 ring

I believe each powder was utilized for a velocity appropriate to its pressure curve.  That fact does not preclude Dan's thesis which was that the key here is that my rifles sweet spot happens to be 1675 fps and that many of the faster powders might work equally well at this velocity.

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CB posted this 18 April 2008

Lefty, that is quite a difference. I have a sweet spot at that 1,675fps velocity. Then another around 1,860fps. I know you said you have a throated rifle which I never shoot, but you have something wrong with fit. The cb is getting bumped out of concentric to the bore with the higher pressures and velocties.

The 1,600fps range is a very forgiving velocity. With halfway decent throat fit, halfway good sizing, about any alloys, and fast rifle powders, 1.5” to 1” and your.88” is fairly reasonable to accomplish. To get down to CBA match competition groups with a production rifle, the cb needs to be fully supported and rest in the throat deep as possible, loaded by the gas check or on into the drive band in front of it. When loaded, the cb should reach 'stop' at the rear of the cb, not the front. Hope this helps...............Dan

What I consider the bible of cb fit was written by the late Ed Doonan in Fouling Shot 135-7; CAST BULLET LEADE. Even in some of the very first Frank Marshal F/S articles about competition shooting, is very good information still pertinent today.

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Lefty posted this 18 April 2008

Dan

Thanks for the input.  My next cange will be to try a different bullet.  I have a mold for the RCBS tapered 200 gr sil bullet.  My hope is that will be a better fit.  I will probably end up trying it both bumped and unbumped.  I guess that willo be a different thread.  It will have to wait though.  I leave for turkey hunting in the morning.

Jim

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