Hi Jim, can you please confirm how many shots you recommend for developing loads.
In Podcast episode 13 you state a 4-shot groups with 4 total groups (16 shot total) is better based on confidence intervals, yet in the BACKFIRE LOAD DEVELOPMENT GOALS WORKSHEET that I just downloaded from the Course, it says to use
● Shoot 5-shot groups at 100 yards and measure centre-to-centre with calipers
● 3 Total groups will be shot and the average of the three groups will be recorded.
Also, I am away to conduct Load development for my 6.5 Creedmoor, what intervals of grain difference would you recommend?
such as 4 total groups of 4 shot groups at 1 grain intervals, with the max load 1 grain back from max powder amount.
We keep bouncing around on the group size because there are benefits and drawbacks to each.
Most of what we do currently is the average of 5 groups with 3 shots in each group. The benefit of this method is you get slightly better data because each group is only looking at the extreme spread so we have more numbers to input.
The benefit of 5-shot groups is a more realistic expression of what would be expected to come out of the gun. However, the last shot or two can get kicked out because of the gun heating up, too. Also, it can be tough to measure groups because the shots disappear into the mass of holes. Also, the aim point can get obscured with too many holes.
There are also other ways to measure such as center mean which really is superior in most ways, but on YouTube May not be that helpful for people wanting to imagine what a group would look like who haven’t seen that measurement before.
So there are pros and cons to everything. But in general, we take about 12-15 shots to get data that is about 90% accurate. There’s a balance between speed of getting good data and also how long it takes and how much ammo costs.