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[WelshShooter]

That is correct, Colin. The "extreme spread" is literally the range of the values which is calculated as the maximum velocity minus the minimum velocity. The standard deviation is the average difference between the data and the mean value. It is possible to have the same Extreme Spread between two strings of shots but have a lower SD. If the spread is always the same for different velocity loading then technically there's less error in the faster velocity load. For example, a 308 bullet can travel around 2,900fps and a 357mag bullet can travel around 1,500fps. If both sets of samples had a standard deviation of 30fps you can say that there's a larger percentage of error in the 357mag compared with the 308. Such a metric is called "signal to noise ratio", which is calculated as the average value divided by the standard deviation. The higher the number, the better.

Apologies for digressing, I use a lot of stats in my day job which I find enjoyable :)

[ColinR]

That is correct, Colin. The "extreme spread" is literally the range of the values which is calculated as the maximum velocity minus the minimum velocity. The standard deviation is the average difference between the data and the mean value. It is possible to have the same Extreme Spread between two strings of shots but have a lower SD. If the spread is always the same for different velocity loading then technically there's less error in the faster velocity load. For example, a 308 bullet can travel around 2,900fps and a 357mag bullet can travel around 1,500fps. If both sets of samples had a standard deviation of 30fps you can say that there's a larger percentage of error in the 357mag compared with the 308. Such a metric is called "signal to noise ratio", which is calculated as the average value divided by the standard deviation. The higher the number, the better.

Apologies for digressing, I use a lot of stats in my day job which I find enjoyable :)

No please digress in the hope of informing me. Can you give me a practical example to better understand or direct me to some reading on the subject that a maths numpty might understand. Presumably SD is not something that can be directly influenced it is just a calculation from velocities obtained. What does SD mean to me as a reloader and can I influence it by any practical means other than ensuring velocities are as tight as possible, or am I completely missing the point?

[WelshShooter]

I can give you a perfect example. Reload Swiss powder, like Vihtavuori, is an extruded stick powder. It has excellent burning characteristics but isn't very useful for metering in powder hoppers like the RCBS Uniflow. If you fill the hopper with powder and start throwing powder, you can feel the powder hopper bind up due to the blockage of the powder. Sometimes you feel some extra resistance, sometimes it binds up completely. I found that if a normal throw gave, for example, 45.0gr of powder, then a throw which bound the action tight could throw up to 45.5gr. A throw with some resistance could throw 45.7gr. Normal throws had a variation of +/-0.2gr.

Ok, so we're talking about velocity but now I'm talking powder?? Well there's a reason behind that. Accurate powder throwing typically controls velocity variation (assuming you've done load development which found a good load with low velocity dispersion but more on that later). If you loaded up a batch of rounds using a powder hopper without measuring every single load you'll get variations in your velocity. The times when the action bound up and threw a 45.7gr charge, these will most likely have a spike in velocity and increase your ES. However, if the action only bound up once in 30 rounds then there will be a negligible effect on you standard deviation. If the action bound up ever 2 rounds, your ES may be the same compared to one loaded every 30 rounds but your SD will now be much higher.

The easiest way to control velocity spread is to weigh and trickle every charge to the correct amount. When I perform load development I always trickle my charge to the target charge I'm aiming for (pun intended). When you measure velocity, you will notice some charges have a large velocity spread whereas some other charges will have much smaller velocity spread. Once I have identified a charge with a low velocity spread, I repeat that loading but I throw charges without trickling as this will represent the repeatability of the loading equipment with less bias on being precise. Why would I do this? I mostly plink and don't shoot competitively so hand weighing and trickling every charge isn't so important to me.

So what is this potential effect and why should you control SD? If you're an F class shooter then you will care about SD but I think they would care more about ES. It's one thing to consistently hit the 9s and V-bull's but it's the fliers in the 4s which are going to ruin your day. A large spike in ES means that either you have an abnormally fast or slower round, which translates into a shift in the vertical direction. The SD on the other hand, will tell you how tight the vertical stringing would be. By the way this is assuming zero human error during the course of shooting :)

I've based the above on being a novice shooter with an interest in maths/science. I don't shoot competitive so I'm more than happy to be corrected based on the experience of others.

[ColinR]

Hmmmm..... I think I understand in that high ES can be the result of one round performing erratically while SD evaluates the overall consistency, such as 9 v bulls and one outer, assuming the outer is not shooter error. If these 10 shots were put over a chrono 9 would have a very low ES and 10 would have a high ES, so the overall velocity of 9 is very close and resulted in 9 v bulls with 1 flyer the result of an inconsistent charge. Alternatively you could have 10 shots in a vertical string with the same ES. Does that sound right?