More powder,same velocity

[PJgunner]

Originally posted by Downwindtracker2
I have been doing load development for a couple odd calibers,264WM and 338.06.The rifles are rebarreled FNs ,one with a Douglas and the other a Shilen,both long throated,longer than then the 30.06 length boxs anyway.Their isn't a lot of data for them,so I have been shooting the groups over a Chrony.One time with each rifle,increasing the charge by a grain,<2%,The next loading shot the same velocity.What's going on here?

Methinks you have passed the max load for your rifle (s).
Let me try to explain. Every powder has a pressure range that within that range it is efficient. If you go below that range, velocity may become erratic, or in the case of the slow burners, you can get a pressure excursion phenomenon, and that's bad as the gun can blow up. However, going the other way, once you have reached the topmost portion of that efficient pressure range, strange thing happen. velocity can either take a bigger increase, stay the same (as in your case) or even be less that the previous load.

Let's say that your cartridge has a max load of 60.0 gr. of powder X. As you approach that top load, velocity stops increasing in a linear mode at say 58.5 gr. That is most likely the max load for that particular rifle. Assuming a 50 FPS increase for each grain increas until you get to 58.5 gr. and all of a sudden, velocity increases only by 20 FPS, or increases, there's no set rule, then that 58.5 gr. load is the absolute max and realy should be decreased by at least one to one and a half grains.

The fact that there was no increase in velocity convinces me of what I've just posted. Be careful.
Paul B.