Component Selection
The important concern with your choice of components is their uniformity and how that will affect our twin goals of minimizing variance of muzzle velocity and of BC.
Uniformity of case volume is somewhat important, but incredibly tedious to check. Many people think they can use case weight as a proxy for volume, but it's an unreliable proxy. Weighing cases is a good way to identify a real oddball that got mixed in with your brass (very rare), but the small weight differences some people use to segregate cases are highly unlikely to represent a meaningful difference in internal volume and thus pressure, velocity and ultimately, trajectory.
Case Wall Variance
The more important selection criteria for brass is case wall concentricity. Although this isn't a widely used selection process, I am confident of its importance to higher scores and higher X-counts. [Some] brands of brass, and military brass in particular, are often terrible with respect to this important determinant of accuracy. If you can't check this dimension, at least play it safe by sticking to Winchester or Lapua brass.
Neck Turning
Once the brass is in hand and selected by case wall variance my next step is turning the necks. Now we're getting down to one of those important steps in reducing SD. Neck tension, the case's "grip" on the bullet, is one of the variables that affects muzzle velocity, and necks with inconsistent thickness will not produce consistent neck tension. Neck turning is the solution to this problem and although it can be a tiring process, there's no real alternative.
Other Brass Prep
Apart from selection by wall thickness variance and neck turning, I trim the cases at each loading. This does two things: it keeps the cases at a safe length, because .308 tends to grow quite a bit with each resizing; and the fresh chamfer on the case mouth is a small but important factor in keeping the bullets undamaged and consistent. I also turn the case mouth into steel wool after chamfering to get rid of any remaining small burrs that might damage the bullet - quite likely an excess of caution, but it lets me sleep better. I don't deburr primer flash holes, having found in prior testing that I couldn't see any change from this process. I also don't cut primer pockets to a uniform depth unless a particular lot of cases seems either unusually shallow or unusually inconsistent
Case Sizing
When sizing the case,we're interested in a two main objectives: ease of bolt operation and consistent headspace. I've written quite a bit about the importance full-length sizing as a standard practice ... Consistent headspace is another important element of good long-range ammunition
Primers and Priming
If you want ammunition with single digit SD, you are going to spend a fair amount of time working with primers - unless you happen to get lucky, but I wouldn't count on luck. Once you have the case necks turned, you can begin working with primers. ... Test your primers by shooting each with a standard load over the chrono, varying only the primer. Fire at least 10 and preferably 20 of each primer type. Look for the one that gives the lowest MV and with a low SD although not necessarily the lowest SD (that will come later). That primer should be the one you use as you develop your load, now looking for the lowest SD load by varying powder charge and neck tension. There's a bit of judgment required here, but a low MV on the standard load and a reasonably low SD is the best indicator of a good long-range primer compared to the others.
