Accuracy vs. Standard deviation

[264shooter]

Am I getting to rapped up in the search for ultimate accuracy with my 6.5-06 Ackley Improved build?

I've tried a few powders with the 139 Scenar, Retumbo, H1000, IMR7828 and Ramshot magnum.
With all these powders the rifle shoots under an inch at 100 yards.
H1000 shows the best accuracy with several sub 1/2" 5 shot groups but the extreme spread and standard deviation are in the teens and twenty's.
I'm comparing this to the numbers I get with my 6.5 Creedmoor and a 308 F/TR gun. I try to get a standard deviation number under 12. This number 12 came from some guys in my club that are experienced 1000 yard shooters.
I use an Oehler 35p chronograph to get my numbers.

I don't think that I'm being anal about this but how good is good enough ?

 

[CK_32]

I think too many guys guy wrapped up in the numbers and paper work.


If you get a 1/2" or 1/2 MOA group that's a dam good load. For MOST shooters that minute of a group has more on shooter skill and consistency than the load it self. Again if your getting 1/2 groups run it. That's a good load.

Also just doing further testing for .1" of better accuracy is wasting barrel life and round count especially if you've already found a 1/2" shooting load.

Unless your bench shooting and losing by .1" stop searching and run it!

 

[Kirkd]

A lot depend on what yardage you are shooting at. You may bot notice a change of hit % at say < 500-600 yards, but will notice at longer ranges. A good read about this and other factors that effect hit percentage is in this book:

Accuracy and Precision for Long Range Shooting: A Practical Guide for Riflemen: Bryan Litz: Amazon.com: Books

I think that I would look at your brass. What is the weight difference? Do you sort by weight? Obviously, if you have a several grain difference between cases, you will get different pressure/velocity. I'm relatively new to precision rifle reloading, but I notice that the closer I get to 100% case full capacity, the lower the SD. If someone with more experience can chime in that would be great. I'm curious if my theory is runs true.

Think the key is consistency from round to round.

 

[Woj8541]

Case capacity (volume), bullet seating depth (also directly related to case capacity), case neck tension, case neck length, and how you size your cases will all play a significant role in what SD numbers you get. Event then, all things being equal, SD numbers also have a lot to do with your rifle's chamber; how it was cut, how much leade there is and so on. Any variances in chamber pressure will dictate SD's. Another thing to remember with SD's is the chronograph variances. I used the 35P for along time and for 95% of the time it's pretty damn close.

If you've got a load that shoots well, don't just take the age-old 100 yard test for granted. That load may shoot it's ass off at 100, but it may not perform well at distance. If you've got a load that shoots well at ALL the distances you want to shoot, then you're obviously all set. If you've got a load that shoots 1/2 minute at all distances you're after, and your chrono says you've got an SD of 18-20, well then I'd say your chrono SD is invalid because we can't argue with downrange results.

 

[CK_32]

If you've got a load that shoots well, don't just take the age-old 100 yard test for granted. That load may shoot it's ass off at 100, but it may not perform well at distance. If you've got a load that shoots well at ALL the distances you want to shoot, then you're obviously all set. If you've got a load that shoots 1/2 minute at all distances you're after, and your chrono says you've got an SD of 18-20, well then I'd say your chrono SD is invalid because we can't argue with downrange results.

Someone said this in another thread I was just in. I see this a lot actually. Hopefully some knowledgable people can chime in too.

As long as you have a good MV, a quality BC bullet and stay above transonic speeds for the most part. A load that holds tight at 100 yards should theoretically group we'll at longer distances as well, shouldn't it?

In my post I'm assuming were taking out all elemental conditions ie wind, temp, pressure to keep this from going full circle in discussion. But how or what would change the effect of a great 100 yard group at say 400 with a 308/6.5 assuming you have sufficient MV and FPS out to the 400 yards and a high or good BC bullet?

The reason why I'm asking is because I'm dialing in a good 100 yard load with 175 SMK at sufficient MV and BC that should hold true at longer ranges. But keep seeing this posted about might not be good at longer ranges. Again assuming you have a great BC, Speed and 100 yard load what aside from the element ie wind and temp changes would effect the bullet since it's already on a good direct path plotting at 100 yards very well should keep true again assuming you keep the speed from going transonic and entering a new elemental factor..


Am I wrong? If so can someone do in debth and explain how this isn't true? Or is that true as long as you have sufficient FPS at those ranges with a good BC bullet that plots we'll @ 100 shouldn't that hold true at farther ranges?


Thanks

 

[Woj8541]

Muzzle velocity is a depreciating asset and it only coincides with accuracy as it relates to how well your chosen projectile is matched to the barrels rate of twist. For example I could care less that my bullet is slow; as long as it's path is predicable and the projectile is stabilized properly I can accurately map it's progress down range. (Yes, in some cases into the transonic and beyond...)

Often the situation is reversed; the rifle groups poorly at close distances and then performs well at distance. This is an indication the rate of twist is not ideal for the chosen projectile, among other things.

Also, with heavy, short and stiff barrels it's difficult to truly find a load at 100 yards that shows the barrels' nodal point. If your game is inside 500, then I'd say you're ok with the 100 yard grouping test, but beyond that it's prudent to test the performance of the load at those distances where the barrels' nodes and anti-nodes will be more clearly defined.


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[CK_32]

Muzzle velocity is a depreciating asset and it only coincides with accuracy as it relates to how well your chosen projectile is matched to the barrels rate of twist. For example I could care less that my bullet is slow; as long as it's path is predicable and the projectile is stabilized properly I can accurately map it's progress down range. (Yes, in some cases into the transonic and beyond...)

Often the situation is reversed; the rifle groups poorly at close distances and then performs well at distance. This is an indication the rate of twist is not ideal for the chosen projectile, among other things.

Also, with heavy, short and stiff barrels it's difficult to truly find a load at 100 yards that shows the barrels' nodal point. If your game is inside 500, then I'd say you're ok with the 100 yard grouping test, but beyond that it's prudent to test the performance of the load at those distances where the barrels' nodes and anti-nodes will be more clearly defined.


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Ok exactly as I thought. I just wanted to be sure in my procedures and understanding of 100 yards grouping and load development all together I'm not missing or not accounting for something with my understanding of 100 yard grouping and what it can show, and not fully agreeing with the blank statement of "If it groups well at 100, it might not group well at longer distances." I'm also not a big proponent of speed until later fine tuning as to stay above a target speed at distances.


Good explanation. Thank you.

 

[Assault116]

It really depends on the distance. Here's how from some quick plug ins in a ballistic calculator assuming a 100yd zero

lets say your bullet @ 175 gr (We'll use the SMK for example) has a bc of .505 @ or above 2800 fps so lets say you get it cooking at 2900 the drop value at 1K is 314.5 inches.

Now lets say you've got your next round just a little slow, so figure that same bullet @2880 fps and you get a drop of 319.6 inches.

These 2 loads will generally have a near exact POI at 100yds

These same numbers at 200 yds equates to .1" well within consideration of a great shooting rifle.

So 20 fps variation at the muzzle equates to 5.1" at 1K, that's just on the numbers alone not considering the spread on your average group to begin with, then the slightest human error @1k is greatly amplified. Add another .5 moa on a generally considered good shooting rifle, and you're now at 10.1" without ANY human error.

The "stacking" of tolerances (Velocity, the gun itself, the shooter, etc) at distance adds up and the longer the shot, the more you're going to be hoping and guessing. Taking a load into the single digit SD area greatly reduces one factor when you get way down range for variation in your shot.

This is of course not considering an accuracy node for your rifle, which coincidentally will many times be a place where your velocity SD is low as well, it is strictly considering velocity alone.

For all these reasons, during load development for long range I look for a load that both produces a good group AND has an SD in the single digits, and the lower ES the better. I don't think I've found a load yet that's got a single digit SD that doesn't group extremely well at 100. Then I take it to longer ranges to confirm that the group holds it's form over long range.

 

[vinconco]

I've gotten to the point where I rarely use a chrono except for basic load setup to get a general idea of the velocity "range". The velocities I use in my ballistic program and drop charts are derived from actual drop tests. As far as SD goes it's impossible to get more accurate than a properly conducted ladder test.

Try setting up 2 or more chronographs in tandem sometime. Which one to believe....... The bullet never lies.