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More often than not you'll be able to see within a few shots if the load is consistent when shooting centerfire loads, either 5 or 10 shots. When I get a couple of big variations I stop and throw the rest of those loads in a box to be disassembled back at home. I don't like wasting components either especially now.
Rimfires are a different story, high end 22lr sometimes requires full boxes to be shot to find out just how many bad rounds there are per box. Some lots of Tenex have only 1-2 per box others are just plain inconsistent without big variations outside the average. It matters when shooting benchrest or silhouette.
I Check point of impacts at 100yds with new lots of 17hmr and write it on the boxes, it can vary a little and sometimes I check the speed just to see what they're running at....just because, I don't really know other than it makes me feel better!
As others have alluded to I'm not sure right now in 2021 is the time to be testing loads, if you have enough components to do so your better off than most of us or not looking very far into the future, it could be 8 years before we see this loosen up just like the last time.
 

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For reliable chronograph information, I usually figure 25 but shoot more to allow for unregistered shots. For group size, I shoot 10-5 shot groups in the spring and 10-5 shot groups in the summer. I going to shoot the ammunition anyway so I don't believe in skimping.
 

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Kosh7528,

For determining Major power factor, you have a different problem than you do in determining group size. Because you have a fixed velocity value to achieve, the closer your average velocity is to that value, the more shots you need to fire to be confident you will pass inspection at the match. The number you need to shoot will get smaller as the average velocity gets further above that line. As an extreme example, suppose the worst extreme spread you've ever seen from your gun is 100 fps. If you shoot one round that is more than 100 fps above major, the odds are, you are done. Sure, you might want to shoot a couple more for confirmation, but you've already beat the odds with that first round. But as you get closer to the line, you have to shoot more to confirm your average is really consistently above it. There's already been a post on this, I believe. I have to sign off for the moment, but take a look for it. I'll look tomorrow, too.
 

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My chrongraph Test Record sheets all, have a 10 shot group pre-printed so
that's what I usually do.
e.g. The same 9mm load in a 4.5" barrel vs a 16" averages 160+FPS


AZB
 

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Discussion Starter · #26 ·
I certainly see your point about out-performing the "Major" threshold by a big margin. In .45 ACP, I could reload to duplicate factory ball and never worry about it, again. Unfortunately, my firearms, my semi-arthritic hands, my shooting budget AND my scores will all suffer unnecessary battering from such a move.
In .45, I guess I'll use data for match rounds and work up, until I'm just over 165 PF, then add 0.1 gr. to the charge, and see how it shoots through other chronographs at matches. I think another batch of ammo, loaded at 0.2 gr. more than "just over 165 PF", may also be a prudent "insurance" move.
 

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I can give you a more systematic approach, but it's a little complicated. USPSA rules are slightly more strict than IPSC. They require three out of six shots to average the power factor velocity number, but stop testing as soon as you have three shots whose average is above the line, so I think it's a good idea to get the first three to pass if you can and keep the add-on shots in reserve. Also, all this is 卤4% because that is how much chronograph variation from day-to-day they allow.

The way I would start is by firing a set of test loads to learn what charge weight your gun needs to reach different velocities in the desired power factor range. Pick a powder for which the published load safe load range for your bullet type includes your target velocity about in the middle of the velocity range. If you know your gun always shoots a little slower or faster than published numbers, you can have that number lower or higher in the published load's range, accordingly.

Fire a load workup in 0.2-grain increments from the bottom to the top of the published range, stopping if you see pressure signs. I use 0.2 grains because that's the resolution limit of most powder scales (卤0.1 grain). If dividing the range up that small does not result in at least 10 rounds, fire two of each or three of each or whatever you need to get to 10 or more. Average the velocities of the duplicate charge weights, if you have any. Plot charge vs velocity results on a piece of graph paper (Excel is convenient if you are familiar with using it, but a pencil and graph paper is just fine). Draw a line straight through what looks like the average data point location to make the estimated trendline (in Excel, use the linear trendline function) so you can see about what velocity each charge weight change produces on average in your gun.

Next, pick the charge weight that looks about right on that line to hit the desired power factor velocity. Load at least 20 of them; 30 is better. Find the mean (average) velocity and standard deviation for that load, even if it doesn't exactly hit the average value you are looking for.

Now we will use a bit of statistical manipulation: Multiply your standard deviation by 0.43. Add the result to the power factor velocity you want to achieve. Subtract the mean velocity of your 20 or 30 round test from this target number to see how much you must adjust the last charge to correct it to the new number. A positive result means you need to add powder. A negative result means you have to reduce that charge. You should be able to find the difference from your chart by locating the two velocities on it and seeing how much the difference in the two charges are. Even if your 20 or 30 rounds did not quite hit the expected velocity, the difference in powder charge for the two velocities on that table should be about right.

The above should put 2/3 or your shots above the desired power factor velocity. In other words, your first three shots would qualify you on average and you have a little room for safety that one of the next 3 shots would get you an average that is above the line. The addition of 0.43 times your standard deviation puts your shots at the point on the bell curve distribution where 1/3 of the population is slower and 2/3 are faster. Using this approach adjusts for whatever standard deviation you achieve. You could optionally make more margin for yourself. Human nerve endings can only detect a difference of about 10% or greater change in recoil energy or about 1.049 times more velocity. The rules only require the competition's chronograph to average 卤4% of its previous day's reading with specified test ammo, so if the chronograph is reading 4% low the next day, you still want to pass. So I would go ahead and take the power factor velocity + 0.43 SD and multiply by 1.04, assuming your chronograph matches theirs. You get to decide your confidence in staying above the power factor number on match day. I suggest you try loading like this and then shoot a number of sets of six over your chronograph to see how many have at least three out the six averaging above major.
 
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I'm lost here. Is the guy asking about match ammo, plinking ammo, self defense ammo or hunting ammo? One of the bad things about reloading, for me anyway, is over the years it's turned into a science. That is only interesting to me to a point and then it get's pointless.
 

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10 was what I always did. I shot a screen or two and switched to 5. :D
 

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I usually shoot about 5 too. If they all are similar in velocity, I'm satisfied.(y)
 

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We don't have 10 shots. We are trying to make 3 shots qualify. If you read post #1, you will learn that what the OP is trying to do is figure out how to qualify for USPSA competition's Major Power Factor load category with the smallest actual power factor he can reliably get away with because arthritis causes him problems handling much extra momentum to buy a safety margin. The rules say the match officials will take samples of your ammo and use your gun to fire three shots over a chronograph, and the average velocity of the three rounds has to be high enough for your bullet weight to make Major. USPSA rules say that if you fail, they will fire a fourth round and then average your fastest three out of the four to see if that passes Major. If that fails, they fire a fifth and again average your three fastest out of the five to see if they make Major. If that fails, they go for a sixth round and again average the top three out of the six. So the bottom line by the rules is you need three out of six rounds averaging at or above Major velocity to have your ammo pass. The one fly in the ointment is they allow their official chronograph to vary 卤4% from day to day, so you want a 4% margin for that.

This really is a probability question that asks what the odds are that at least one of two strings will stay above the velocity required to make your bullet weight pass. A lot of information is missing, making a solution more challenging to come up with. I know a lot of people don't like all the math or statistics and see it as brain clutter, but that leaves you with only one other way to solve the problem: Develop a load you guess might work and fire enough groups of three to learn what percent will fail to pass and decide whether that is acceptable to you or not or whether you can lower the charge further. Otherwise, you put up with the math and save a lot of bullets and powder and primers.

I provided the simplest approach to the math that came to mind, but it still leaves open that we don't know the accuracy of his chronograph. There are a couple of other issues to be resolved, like how much risk of his 6-shot test allowance is he willing to take? It will never be zero. The answer will change if 95% success in passing is acceptable or 99%, or whatever. The lower that percent, the higher is average velocity will have to be. Since the competition allows its chronograph to vary 卤4% from day to day, his actual velocity requirement is 4% higher to be sure a slow day for the chronograph doesn't get him. At 0.43 standard deviations faster than passing you have an even probability that two of your first three shots will be above the line.
 
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As I recall, the instructions with my chrony indicated 10 shots if your are wanting to calculate parameters like standard deviation or expected range of velocity variations for a given load. The chrony I used has several registers that record 10 shots in each register, so it was pretty much designed based on the expectation of shooting 10-shot strings. Based on the OP's stated need, I think he just wants to assure no shots are less than some value, so 3 shots might do.
 

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Shoot a ten shot group.

Take the standard deviation of the ten shots.

Multiply by 6 (Six-Sigma).

Bump your average velocity by the six sigma value and you are done.

Six Sigma gives you 3.4 rounds out of one million that will fail to meet your goal.

I designed and built my own Chronograph in 1992 and am still using it today. One of these days I will design a replacement since the programming software is 16 bit and I don't have a computer it will run on any more.
 

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How many rounds, per firearm, make a representative sample for a particular charge weight/propellant/projectile/primer/cartridge overall length combination, when working up a load for a specific application?
Obviously, 50 to 100 rounds would give LOTSA information on extreme spread and variance, but that's a lot of components to use, especially in initial stages. Five or ten rounds seem(s) too small, but might tell the shooter whether there is potential (for hazard OR superior performance), depending on the round of interest.
Let's say we're trying to make "major" for USPSA with a .40 S&W or .45 ACP. Bullet Weight x Muzzle Velocity, divided by 1000, must equal or exceed 165. Because Murphy's law is our constant companion, we want a 3% margin of comfort, in case their chronograph clocks differently than ours, so our goal is a PF of 170. We need 850 f/s with a 200 gr. projectile, or 920 f/s with a 185 gr. projectile.
CERTAINLY, as we shoot a particular load over time, we'll learn whether it's what we're after. But when we're at the bench, trying to balance economy of components against certitude of results, what number of rounds for a certain (untried in OUR firearm) load combination seems a valid compromise?
As a B.A.G., I'M inclined to think that three full cylinders (15 to 24 rounds) is enough for most revolvers, and 20 rounds is about right for most auto pistols. There's nothing MAGICAL, to me, about 3 full cylinders being about right for revolvers. My decision about this amount is completely arbitrary, except that, intuitively, it seems reasonable.
Thoughts?
6 a cylinder full.......in both rifle and handgun I start loads near the target velocity (I'm not a max speed junky) and go for accuracy first, then chrono to see what it runs at.......it dont matter how fast its going if it dont hit what you are aiming at.....
 

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The big advantage of a Chrono in my opinion is not just that you see the speed of the bullet, but you see the spread of the speed. The standard deviation is a very good indicator of how much spread in drop and wind drift you will see at greater distances. You can not see this on a target at 100 yards.
 

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Discussion Starter · #36 ·
6 a cylinder full.......in both rifle and handgun I start loads near the target velocity (I'm not a max speed junky) and go for accuracy first, then chrono to see what it runs at.......it dont matter how fast its going if it dont hit what you are aiming at.....
I'M not a "max speed junky", either, but I'd think it difficult to develop an accurate load without knowing the amount of variance one has with a particular component combination. Additionally, if a certain threshold is not met or exceeded, accuracy becomes quite secondary quite rapidly. To use your colorful parlance "it don't matter how accurate it is if it don't make "Major". I'd certainly prefer a load that groups 6" at 50 yards but consistently registers 170 PF to one that exceeds the 165 PF threshold only 90% of the time and groups 2" at the same distance.
 

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Shoot a ten shot group.

Take the standard deviation of the ten shots.

Multiply by 6 (Six-Sigma).

Bump your average velocity by the six sigma value and you are done.

Six Sigma gives you 3.4 rounds out of one million that will fail to meet your goal.
He doesn't need all his shots to be above the Major Power Factor line. Only his three fastest rounds out of six need to have an average that makes major by the rules. (Note: I had to reread the rules and revise my previous two posts, as I'd misread the chronograph tolerance as an acceptance tolerance for velocities.) So, he could fire ten sets of six rounds and find the standard deviation of the ten resulting mean values and if he really wants to have 6蟽 level of certainty he would multiply the SD of those mean values by six, then add that to 1.04 times the major power factor velocity to allow for the chronograph tolerance. But he's trying to minimize the recoil impulse for his arthritis, so I expect he's willing to accept that every once in a while he only makes minor. He'd probably be happy with a 95% probability of passing, so multiplying the PF velocity by 1.04 and then adding just 1.65 mean velocity SDs to that would get him there. 2.33 would get him to a 99% probability of passing.

A simpler approach would be to use Standard Error, which would, in this situation, be best calculated by multiplying the ES of his 6-shot groups by 0.161. The standard error is the expected standard deviation of the mean from one sample to the next. It is just 蟽 (the population standard deviation) divided by the square root of the sample size. For samples of 7 or fewer, dividing extreme spread by zye of n ( 尉 (n) ) is considered a better estimator of population standard deviation than the usual chronograph sample SD calculation is, and 0.161 is 尉(6)/鈭6. But since the outcome is dependent on just the two most extreme velocities, you probably want to fire at least five sets of 6, multiply the extreme spreads of each by 0.161 and use the average result to multiply by 1.65. So we are still at 30 shots total.
 
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Been a long long while..have not worrried about riding the edge of major vs minor in like forever...but how many rounds do 鈥渢hey鈥 shoot to test for 鈥渕ajor鈥? Is is 鈥渙ne and done鈥...3 shot average...or 5 shots?..local or major shoot?

Can shoot a long long string of shots鈥..but look it over for 鈥渓ady luck鈥...is there any 3 consecutive shots in your long string that would fail?...any ONE shot鈥.any 5 consecutive shots?
 

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Ribbonstone2,

From my post #31 revision, which I had to do after re-reading the USPSA rules a little more carefully:

"USPSA rules say that if you fail, they will fire a fourth round and then average your fastest three out of the four to see if that passes Major. If that fails, they fire a fifth and again average your three fastest out of the five to see if they make Major. If that fails, they go for a sixth round and again average the top three out of the six. So the bottom line by the rules is you need three out of six rounds averaging at or above Major velocity to have your ammo pass. The one fly in the ointment is they allow their official chronograph to vary 卤4% from day to day, so you want a 4% margin for that."​
 

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ok..."YSSUP" rules.

If they are going to accommodate living that close to the edge, may as well take advantage of it.
 
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