Why are certain grain bullets recommended?
So a few weeks ago I was wondering what the rate of twist on my rifle is and couldn't find it anywhere for my cartridge, specifically. I went ahead and contacted Remington via e-mail and got this at the beginning of the response:
"The rate of twist for a .308 WIN 1:10 Faster Suggested Grain weights = 168,165,180..."
Why are they essentially recommending any standard factory .308 loading except the 150 grain?
I tend to shoot 150 grain soft points, since they're faster than 180s and the latter is unnecessary for shots within 200 yards. I consistently get MOA groups at 100 yards (with the same load, I used to get cloverleaf groups; I think it's because I need to do a thorough copper cleaning).
So I'm likely to keep doing what I'm doing, but I still want to understand conceptually what's going on. Why would I supposedly get better performance with a heavier load? I get that at longer distances higher ballistic coefficient can be a huge plus, especially on larger game, but that's strikes me as a general rule of thumb---not something that is self-evidently true specifically for 22" barrels with a 1:10 twist.