1000 Yard .308 Load Development
by Germán A. Salazar
How To Fail
The Magic Numbers
The single biggest challenge in long-range shooting with a .308 is keeping the bullet above the transonic range as it reaches the target. Once it dips into that velocity range, accuracy will be compromised and in many cases the bullet will become unstable and a potential danger to the target marking crew. In light of the foregoing, our primary concern is keeping the bullet out of that transonic range and the first magic number that we must keep in mind is 1.2 mach (1.2 times the speed of sound). The speed of sound through air varies with atmospheric conditions; accordingly, there is no absolute velocity for 1.2 mach, it will depend upon the conditions present at the range where you shoot.
The second magic number is the muzzle velocity that you will need to have in order to ensure that your bullet remains at or above 1.2 mach at 1000 yards. We'll discuss that next.
Working the Numbers
If you aren't already familiar with it, now is a good time to go to JBM Ballistics and get acquainted with its functions. There are many other ballistic programs, of course, but I use JBM because it is accurate, complete, online and free. Once you're there, click Ballistics, then Calculators, then Trajectory; that will put you in the right place for the calculations needed for this project. Two key atmospheric inputs to JBM are your range's altitude above sea level and the temperature; humidity and atmospheric pressure are far less significant. Google Earth is a very useful tool for determining a range's altitude; temperature will, of course, vary depending on the season. However, because cold air is more dense and thus increases drag, MV must be higher in cold temperatures to ensure having 1.2 mach at 1000 yards. Your calculations will be adequate if you use the lowest temperature you expect to encounter during a season as your standard temperature.
JBM has a very complete library of bullets which includes any realistic bullet choice for long-range shooting with the .308. When available, I recommend using the bullet choices with the (Litz) designation; these include Bryan Litz's very accurate BC numbers. Once you enter a bullet choice from the library, you can ignore the other bullet input fields such as BC, weight, etc. as they are all embedded in the library choice.
As we've discussed in many previous articles, a good long range load will typically have a low standard deviation (SD) of muzzle velocity, so this is something to look for. If you are above your magic number for MV, are seeing no signs of excessive pressure, have a relatively low SD (10 or lower) and are seeing good accuracy, you're done. From this point forward, your job is to shoot to the load's capabilities.
In Part 4, we'll discuss how case capacity and seating depth can affect your loads and pressure.