From what I understand they find a "plateau" where the velocity does not vary much as the grains increases.
They load for the powder weight at the center of this velocity "plateau". So even if the metered powder is slightly low or high the velocity won't be affected as much
tim, for the past couple of months now i've been reading about a lot of the different accuracy theories, specifically the ones that relate to load development (e.g. OCW, OBT, Ladder, Satterlee, etc.). one of the common themes i'm seeing is that, in order to get useful results from your load development there are 3 things that a lot of people, especially new reloaders, ignore.
1) the rifle needs to be very precise. if the rifle is incapable of putting tiny groups on paper, then it is almost a waste to do load development for that rifle.
2) the shooter needs to be able to put tight groups on paper. if the shooter can't put tight groups on paper, then there is no way to really compare results of different loads. a lot of the guys that shoot long range or benchrest stuff are able to mostly take the human equation out of it. it's interesting when you go to KHSC and you see people who have terrible technique and are not practicing good shooting fundamentals.
3) the reloader needs to understand the importance of each step in the process and needs to produce consistent loads. if the loads aren't consistent, then it almost doesn't matter good the rifle or shooter is since the results will not be conclusive.
here's an example:
i recently learned the importance of consistent neck tension. i did some load development for a 308. i was able to find a load that seemed very accurate, but i was strangely getting one flyer out of a 5-shot group.
in all 3 pictures below, the load is exactly the same. two of the pictures were from two range trips where i shot a 5-shot group to test the load. note the one flyer of the 5 for both groups. also note that the center x-ring is 1" in diameter and the rings are graduated in 1/2" increments.
when i got home, i used a digital caliper to check the neck diameter of the brass i was using. a portion of the brass had a neck ID ranging from 0.296" to 0.301". ideally for a bolt action you want the neck ID to be around 0.306" or somewhere around there.
after i measured out all of the brass, i made more rounds of the same load. on the third trip, i let my friend have some time behind the rifle. the third pic is her 7-shot group. I would have let her squeeze of 3 more rounds, but the buzzer rang and we had to head out. this was a different set of stickers, which are 1" in diameter. i drew a yellow circle that is 1" in diameter (same size as the sticker) to show her that her group was easily sub MOA.
anyway, moral of the story is: in order to have useful results from load development, you need an accurate rifle, a good shooter, and good reloading practices.
edit: before klapp asks, yes, these were shot at 100 yards.


