First time calculating STD Deviation


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mwsenoj
January 7, 2012, 12:30 AM
I used my chrony today for the first time to find the standard deviation of my .223 rounds. I dont really know what a good std deviation looks like for this round. My numbers are as follows:

Std Dev: 25.64
Mean:3230
String: 19 shots

Is this any good?



* Using a plasti-Lee "perfect" powder measure that just got me into reloading last year and I am shooting from a 20" remington 700. Having some fun with this rifle and want to make it and myself better.

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helotaxi
January 7, 2012, 12:57 AM
That's a little on the high side. Normally you want an extreme spread about where your SD is. Statistics says that 95% of your shots would be within 2 SD's of the average both above and below. That basically means a 95% confidence extreme spread of about 102fps. Like I said, that's pretty high. With a good stable load and consistent neck tension, single digit SDs are the norm with extreme spreads in the 20fps range or lower.

Action_Can_Do
January 7, 2012, 06:46 PM
I've seen factory ammo do much worse. It isn't that unusual to see an extreme spread of 90fps in some brands.

mwsenoj
January 7, 2012, 07:02 PM
My goal for the time being is to shoot target and maybe coyote out to 500 yds. I calculated that my std dev will put me ~ +/- 2" @ 500yds which isn't unworkable, but will make the shot less likely to hit my targets which range from 5" to 9" in diameter. Do you guys have any suggestions to help with this? I am shooting 53gr Hornady VMax and using H-335 @ 25.7gr and I have been using IMI brass that has given me some problems with the shoulder and case head size but it was new and free.

helotaxi
January 7, 2012, 08:42 PM
Neck tension has to be consistent. Ignition has to be consistent. Burn has to be consistent. You could neck turn all your brass for consistent neck thickness. After that anneal the necks. Finally try different primers or a different powder.

Mccarty
January 7, 2012, 08:56 PM
I am not familiar with the lee perfect powder measure - is it actually weighing each load, a dipper or just a powder drop? First thing I would check is to make sure every charge is identical. If this is being done, I would then verify seating deths are identical for all loads.
Some additional quick things to check are the crimp and expander. If you are using a crimp, try it without or a very light kiss. Are you, lubing the inside of the neck prior to sizing (assuming you are not sizing with bushings). Sometimes the expander ball can pull back through the neck and stretching it a bit and losing seating tension. Some military brass can have some thick neck brass. You may even want t try some factory ammo and then reload some brass with known history.
Powders can also help settle down some some SD and ES.

Experimentation is the key to finding what your rifle likes.

Do you know what twist your barrel is? Trying some heavier/longer bullets may also help out with the 500 yd goal in terms of drop, drift and energy.

Just some thoughts

helotaxi
January 7, 2012, 09:12 PM
Trying some heavier/longer bullets may also help out with the 500 yd goal in terms of drop, drift and energy. If you're not familiar with the bullet he's using, it's in the top five most efficient varmint bullets I've found. The benefit that it has over the heavier bullets is about 500fps when loaded to the same pressure and fired from the same test barrel compared to the 75gn BTHP from Hornady. It flies 7" flatter to 500yds than the 75gn pill it is still going 100fps faster at that range and, unlike the heavy match bullets, it will explosively expand at that range and dump all its energy on the target. It also doesn't present a ricochet hazard either. It really is a fantastic little bullet for long range varminting.

Mccarty
January 7, 2012, 11:06 PM
I was thinking more of windage, rather than drop. I regularly shoot to distances of 500 and 600 yards in highpower and midrange events. There is a clear accuracy advantage with a higher BC bullet in the wind . I am very familiar with the vmax bullets and absolutely not knocking it. Simply making a suggestion if his twist and chamber will support it to maximize his potential.

Mccarty
January 7, 2012, 11:38 PM
... I do see that I included drop in the sentence. My mistake.

I did a little checking into the math using the Berger ballistic calculator:

53 gr Vmax has a BC of .247 and using H335 powder should yield approximately 3200 fps. Assuming a 100 yd zero and 10 mph cross wind the bullet should drop -56.75" and drift -38.37". It would be traveling at 1511 fps at 500 yards and carry 269 ft/lb energy.

75 gr Amax has a BC of .435 and should yield approximately 2700 fps. Assuming a 100 yd zero and 10 mph cross wind the bullet should drop -61.43" and drift only -23.65". It would be traveling at 1756 fps at 500 yards and carry 514 ft/lb energy

The heavier bullet even going slower at the muzzle will not shed velocity as fast. At 500 yes it only drops 4.68" more than the 53 grain bullet but has 14.72" LESS drift and has almost 2x the energy (514 lbs). As the distances increase the advantages grow.

Twist rate is critical however. I have a Remington 700 in .223 with a 1:12 twist barrel. I tried my service rifle 75 gr Amax load in the 700 and could not get on paper at 100 yds (with a 50 gr vmax it will almost put them in the same hole). I took the 700 and the 75 gr load to the 25 yd line and discovered they were tumbling and impacting sideways at 25 yds!

denton
January 7, 2012, 11:52 PM
At the risk of offending everyone, a standard deviation of 25 is nothing to be worried about unless you're doing long range shooting with a 1/2 MOA or better rifle.

It's not at all unusual for commercial ammunition to have an SD around 35. I've tested "high energy" 30-06 ammunition that does indeed deliver another 100 FPS but also has an SD around 60. In their calculations, SAAMI assumes that the standard deviation of MV is 4% of MV. So their "norm" is that a 3000 FPS cartridge has a standard deviation of 120 FPS.

My 223 bolt action will do 5/8" five-shot groups at 100 yards all day long with ammo that has a 25 FPS standard deviation.

You can indeed get your SD down into single digits without a huge effort. Sort your bullets by weight, routinely anneal necks after about 4-6 reloadings, etc. But this is not a productive pursuit.

Variation does not add linearly. When there are many sources of variation combined, as there are when you shoot, the largest source will almost entirely determine the total variation. The math probably isn't something we want to do on this forum, but trust me on this. I do this stuff for a living. If your MV variation is the largest source of error in your marksmanship, you have my admiration. If it's not, there is no point in worrying about it. The one and only way to improve rifle accuracy is to find the single largest source of variation (usually you!) and work on that. Working on the second or third most important source of variation will generally produce almost no improvement in the overall picture.

A few years ago I did an article comparing various tools for measuring powder. To my utter astonishment, with ball powder the Lee Perfect Powder Measure has less repeatability error than a good balance scale, which in turn has less repeatability error than a digital scale. Individually hand weighing your charges will not get you more consistency than that little $25 powder measure. You could have knocked me over with a feather when that result came up.

mwsenoj
January 8, 2012, 03:01 AM
My rifle is a rem 700 SPS tactical with a 1 in 9" bbl.

The claimed BC from Hornady is .290 instead of .247

And, Denton, I like what you have to say :) I am most certainly the biggest problem with my shooting at the moment. I tried to attend an Appleseed event in October but it didn't work out. I have been shooting American Eagle .223 from Walmart @ $6 a box of 20 but I have really been enjoying reloading lately so that is why the std dev ?s come in.

helotaxi
January 8, 2012, 06:52 AM
53 gr Vmax has a BC of .247That is the 53gn AMax, not the VMax. The VMax has a longer ogive profile and a higher BC of .290.

75 gr Amax has a BC of .435...And won't fit in the magazine of most repeaters. I was comparing to the 75gn BTHP from Hornady (BC .395) which can be loaded to magazine length and using the factory ballistics tables on the Hornady website for their Superformance .223 ammo for velocity from a 24" barrel. Yes, even the BTHP will exhibit slightly less drift (about 4" at 500yds) but if the SD of his loads is on the high side, the drop is actually of concern. Also the difference in wind drift is almost negligible until 300yds. And while the BTHP carries more energy downrange it isn't a varmint bullet and isn't very well suited for smaller varmints. Yes, I know that people use them for that but the VMax is considerably better for the reasons that I listed above.

Twist rate is critical however. I have a Remington 700 in .223 with a 1:12 twist barrel. I tried my service rifle 75 gr Amax load in the 700 and could not get on paper at 100 yds (with a 50 gr vmax it will almost put them in the same hole). I took the 700 and the 75 gr load to the 25 yd line and discovered they were tumbling and impacting sideways at 25 yds!Too bad you wasted those bullets. The 75gn AMax needs a 1:8 twist to be stable. I thought that was very well known. The 75gn BTHP is a lot more flexible since it generally only needs a 1:9 which has become a fairly common twist in bolt action rifles (except the Remington SPS Varmint which I why I passed on one last year even though the price was fantastic and it was even left handed).

Denton-- Good input. Brian Litz discusses this a good bit in his book, specifically with regard to wind doping. The reality is that high SD doesn't really become a significant issue until you're out in the 1000yd area and are a sub MOA shooter at that range.

Mccarty
January 8, 2012, 10:23 AM
You are correct - I grabbed the incorrect BC for the 53 gr. The theory and application of a higher BC bullet at longer ranges still applies and was just a suggestion. I was not suggesting the abondonment of the 53 gr Vmax by any means. I also was not suggesting a particular bullet, just one with a higher BC. The OP stated for use out to 500 yds. I did not say that the 53 gr vmax would not make it, just that it is more succeptible to wind over long distances. You said it yourself that drift was negligible up to 300 yds -This is why my suggestion for a higher BC bullet at 500 was suggested (never implied as necessary)

As far as wasting a handful of bullets to settle my curiosity, I am fine with it - I have thousands. I am well aware of the twist recommendations for stabilization. I was just making sure that if he chose a heavier bullet that he had the means to stabilize it.

As far as the original topic of SD is concerned, I definitely agree that it is not everything as far as accuracy goes. Lots of times when working up a load we see a larger SD or ES than we would like, but the groups proved to be very accurate. On the other hand, I don't think that a quests to reduce the SD and ES are futile. I try to work on all aspects of accuracy whether it is the rifle, the ammunition, position, environmental factors or the various other things that come into play. Sometimes, even if only for the understanding. It is kind of similar to the quest for more hOrsepower, torque and handling in a car. Where you draw the line is up to you

denton
January 8, 2012, 11:23 AM
I ran the math for an article in Varmint Hunter, Oct-Dec 2007, page 91.

If you have a 1/2 MOA rifle and are shooting at 500 yards, and if you have zero variation in your MV, your rifle will print 2.5" circular groups at 500 yards. If you then add in the variation from 15 FPS MV standard deviation, your groups will print an oval 3.2" tall and 2.5" wide. If you increase the MV standard deviation to 24 FPS your oval will be 2.5" wide and 4" tall. That's not a serious impairment.

If you have a 1 MOA rifle, the vertical stretching of the oval you'll print is barely noticeable with an MV standard deviation of 24 FPS. The natural variation of the rifle is enough to dominate and mask the effect of MV variation, because variation does not add linearly. When several sources of variation are combined, the largest source almost always sets the total variation.

With either a 1/2 MOA rifle or a 1 MOA rifle, in the real world, other sources of variation are likely to mask both the natural variation of the rifle and the MV variation.

You cannot improve a process by working on anything but the largest source of variation. Working on absolute consistency for a minor source of variation may be theraputic, but it will not noticeably improve the process.

Ledhore
January 9, 2012, 12:25 PM
denton - you make an excellent point that is often overlooked!

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