desiredusername,
If you are talking about High Power match rifle shooting seat the bullets so the rounds will feed through your magazines. Generally 2.260" +/- .010" OAL depending on your magazines. Usually only your long range ammo will have the bullets seated out close to the lands which requires single loading. But for long range (500/600 yards, actually "midrange") you will need 75/90 grain bullets. In your 9" twist the Hornady 75 grain A-Max should work ok. I don't think the Sierra 80 grain bullets will work with it, and I'm not sure about the Sierra 77 grain bullets. I've never tried any of the Sierra bullets in a 9" twist and GENERALLY anything over the 75 grain Hornady needs an 8" twist, and the 90's like a 6.5" twist.
How far off the lands (for mid and long range) depends on the throat length on your barrel and where your particular barrel likes them seated for best accuracy.
Most common powders seem to be N135, N140, 4895, RL15, but lots of others will work. For the heavier bullets at long range N540 is popular. I have used 748 with some luck when I shot the 69 grain bullet many years ago, never tried 2520. I think if I had to pick one powder for High Power it would be N140 or RL15.
Good luck and good shooting,
Phil