FPS spread

[Destry34]

What is a good avg spread? I have made up 3 different loads to make pf and have a pretty big spread. I know some powders are more consistent on the chrono, just wanted to make sure none of these are just off the wall

84fps - Solo 1000

57fps - WSF

36fps - TG

[Sarge]

You are looking at the ES or extreme spread. These numbers can get pretty high if you have a random very low or very high velocity on one round. Your chrono should give you an SD or standard deviation reading as well. That is the number that means the most aside from Power factor of course. SD will tell you what the typical difference is among all the shots in the string more or less. Single digits are ideal but low teens are pretty good.

Also, it takes quite a few strings over the chrono to really get a feel for what each load is doing. 10 or 20 are not nearly enough data. Load 100 each and chrono them all while shooting at a target beyond the chrono. This way you see how well they chrono and how well they group. Load development is tricky in that you want to make a good comfortable PF yet you need a good tight shooting load as well for accuracy.

[Destry34]

Thanks for the explanation

[pjb45]

Are you using the same brass. Different brass can have an effect on your FPS. Older brass v. newer will do the same thing.

I sort of agree with shooting a lot to get a decent average but I also shoot in three shot groups. If you are loading match ammo, I want to mimic the Chrono procedures. There are some statistical black magic associated with this mentality.

Over the course of a high number of shots, you could have three shots which are below the average. So you could go minor. So the chrono procedure would be invoked. So now, you use four more shots, you have the top three for your average. My chrono allows me to down load to a file which I can analyze in Excel. I check my individual strings then combine the strings into one group. I also order my shots to see the lowest to the highest FPS. If I get 10 or more (out of 50) velocities below my floor I will reassess the whole sample.

This allows me to look at strings of fire plus the total aggregate of all strings.

Standard deviation is your friend. Small is good. Look at your high and low also.

I usually kept my PF at 170 with various range brass-once fired. I have never gone minor.

I changed my recipe sometime ago, I started using a 166-167 PF at home. It has not failed to make major at Nationals or an Area match yet and it is a very soft shooting load. I am very comfortable that this will always make major if there is not a dramatic drop in the outside temperature.

[Skydiver]

Knowing your standard deviation is a big plus. You can determine how much of a risk you are willing to take if you want to play games on the chrono stage.

Check out the picture at: http://en.wikipedia.org/wiki/File:Standard_deviation_diagram.svg

The center line is your average and the 50% mark. It means that about 50% of your rounds will be above the average. The first line to the left of center is 1 standard deviation below your average. It also means that over 84% of your rounds will be above your average minus the standard deviation. The -2 standard deviation line means over 97% of your rounds will be over your average minus twice your standard deviation. So if you make sure that average minus twice the standard deviation is at 167, you never have worry about the chrono stage tagging you as minor, given the way the chrono rules work and the way probabilities work. You could make your margin slimmer by targeting the average minus one standard deviation to be at 167, and I believe the probabilities are still in your favor.