48.7 grains of RL26 with a 142 ABLR is giving me 2977 ft/s out of a 22" barrel. The primers are flattening but still have a rounded edge, no cratering and a normal bolt lift. They hold around 3/4" which is just fine for my purposes.
This is exactly what I have been saying and I "tried" to have this conversation with Sierra who kept stating "we believe it will stabilize". Then after a while they started blaming the calculator and said in their tests it was fine down to 2200 ft/s (muzzle velocity) out of a 10 twist barrel...
48.7 grains produced 2909 ft/s. The primer was flattening but still had a rounded edge to it. There was no cratering and the bolt lift was normal. 3 shot group came in at 5/8" but just about every other group on the way there did too.
I'm guessing this is probably a good place to stop.
Hodgdon shows 72.0-77.0 @ 3.340 for a 300 WM and probably for good reason. I would start there and work up as you find what suits your particular setup
I too have a 300 WM (Sendero SFII) and tried to load them close, also like you. I backed it way off to the tested COAL of 3.340 and BINGO. It just worked there and I am shooting them with 77.0 grains of H1000 at 2886 ft/s so hopefully your speed of 2040 and 2060 are typos. Mirage can be an...
The BC for my hunting conditions is compromised 10%, placing you in the .440's. E-mailed Sierra about it, which they swear it will stabilize in a 10 down to 2200 ft/s (y) I will agree that it may still shoot accurately but you are not getting all the benefit of it. There are a lot of 10 twist...
These bullets were not designed for us “10” folks. Even the 9.25 can be a little iffy. That is not saying they will be unstable. Good groups may still be attainable with both but you won’t be getting that .490 BC either