Originally Posted by 3006savage
It does not seem based on all the responses like velocity is a consideration if one is on the fence trying to decide between two twist rates.
Now that is not the question you asked originally. You asked if there is a rule of thumb.
But to answer the last question, look at the 240 SMK, Sierra says you should have a 9 twist. Why? It was origianlly designed for 2600fps for NRA LR comp in the 30-338 and 300 Win which were the two LR comp calibers then.
Yet today we normally run the 240 in 10 twists at 2800-2900 with larger cases such as 300 Wthby and 300 ack imp and smokin groups. So is it twist, stability or what? I just know that you do not have to use a 9 twist for sure. I also know it does effect MV and pressure from what I have seen using a slower twist.
I have found that slower twists allow more MV with less pressure and absolutely no loss of accuracy out to 1000 for sure, but again very barrel dependent.
As for all the subtle questions you asked, the answers would only apply to that one barrel, chamber, twist throat combo and what is the significant impact. you could do all that testing but what does it get you that has real applications.
Barrels are like twin women, they may look alike and have same parents, but boy are they ever different.
To me that amount of endless testing is kind of like "pole vaulting over mouse turds". You can do it but so what?