RE: Why are some calibers more accurate?
Accuracy is influenced by a lot of factors. Factors that aren't so important at 300 yards can become very imporatant at 1000 yards? Such at BC of the bullets, and velocity (and velocity retention for some, since it is thought that the transition from supersonic to subsonic can have a negative effect on accuracy). However, the one thing you will notice is that almost all cartridges that are regarded as highly accurate are very consistent, sometimes shooting loads with and ES fo 10 fps or less. This consistency tends to favor shorter fatter cartridges such as the PPC's, the BR's, the 308 and it's kin, the 284 and it's offspring, and recently some have been using the WSM and it's offspring with a bit of success (at least at the 1000 yard matches). This isn't to say that longer cartridges can't do it, with the main point being for the 300 Win Mag which is one of the more popular 1000 yard calibers around, and has a reputation for being easy to get consistent. This consistency affects accuracy positively in several different ways, first since the bullets are moving at very close to the same speed, they are exiting the barrel at the same point in its vibration every time (this is harmonics, which is a whole different discussion), thus with the bullets exiting the barrel with the barrel very close to the same point every time, and with bullets moving very close to the same speed, the impact point should be very close! Thus making for a smaller group. I hope I haven't confused the issue any, but this is my take on why some cartridges are more accurate than others.
Just my 2 Cents.
Gordon