In the article, "What is a ladder load?" we made a stab at how to develop a load for a particular powder and a particular bullet. Then we linked to some google spread sheet that you can copy and save in your own google drive so you can edit.
Here's a sample set of charts from the data we captured. The goal is to understand each of the 5 sections and what they tell you. As always, the data is fictitious but realistic. Most of the time, the randbetween(x,y) function is used in the spread sheet to generate the data. Our ultimate goal is precision (how close our shots are to each other) and accuracy (how close we are to the target).
Section 1: Velocity Differences Using Large Rifle Primer (LRP) and Large Rifle Magnum Primer (LRMP)
Some powder/bullet combinations might work better with a normal primer or a magnum primer. The team here uses small rifle primer versus large rifle in their tests. The primary goal in capturing this data from your ladder load and your chronograph is to determine how consistent your velocities are and if there is a pressure/over velocity issue.
Section 2 - How consistent are these loads?
Here we are looking at the data and asking, "How close are the velocities per powder measurement?" For example, the 5 rounds with 35.0 gr of IMR 4895 have a velocity difference (spread) of 32 fps with lrp and a spread of 49 fps with lrmp. Again, fictitious data generated by a random function in the spreadsheet.
To consider: a goal for reloaders is to get their spreads below 10 fps. Differences between high and low of 32 fps or 49 fps are acceptable when beginning. But those numbers should decrease with experience. In case you might have forgotten, there are items in shooting you cannot control: temperature, wind speed/gusts, wind direction, barometric, humidity, and elevation. What you CAN control is how much variation is in your rounds in a given batch. The greater the consistency in your loads, the more predictable and accurate you will have opportunity to be.
The measure of variation in our case is provided by Standard Deviation (stdev). The lower the stdev, the better. Lower means that the individual rounds are getting closer to 'identical.'