Cartridges: 1000 Yard .308 Load Development

1000 Yard .308 Load Development
by Germán A. Salazar

This article is Part 3 of a series on loading the .308 for Palma or other long-range shooting disciplines. The first part was Palma Preparation and Loading in which I covered some of the principal objectives of a .308 load for Palma shooting. The second part, Loading the .308 for Palma Matches gave detailed information on component selection and my approach to meeting the goals described in the first article. In this article, we will examine the essential requirements of a long-range .308 load, and we'll emphasize some important cautions for the reloader new to 1000 yard shooting.

How To Fail
My observation of the reloading failures of many shooters, particularly those new to long-range shooting, leads me to the conclusion that most of those failures are the not the result of too little information, but rather of too much information combined with the recipient's inability to filter it in a meaningful way. Shooters are a friendly and gregarious lot, there's never a shortage of advice for the new shooter at a match; then there's the internet with it's almost endless supply of advice, usually from anonymous forum posters whose experience and qualifications may well be limited to pecking at a keyboard. The new shooter, who is asking about a specific cartridge, the .308 for purposes of this discussion, often believes that all of this information is: a) good; and , b) applicable to his rifle - he is usually wrong on both counts. Frankly, much of the information that gets passed around in person and on the internet is ill-informed and dangerous and there is enough variability in brass, chambers, barrels and components that one person's experience is rarely directly transferable to another person's reloading.  The only thing more dangerous the mindlessly copying someone else's load is taking bits and pieces from different loads and thinking they'll somehow produce a usable load.  We'll discuss this at greater length in Part 4 of this series, but this is exactly what leads to many accidents.

Although these observations are applicable to all ammunition reloading, loading the .308 for 1000 yard shooting presents a special set of problems and leads to even more misinformation and dangerous practices than we see in other cartridges. To be as blunt as possible about it, you can't rely on anyone else's information when reloading the .308 for 1000 yards; you must develop your own loads in your own rifle with your own components. This level of reloading is very critical, even some of the semi-commercial ammunition being sold for this use exhibits dangerous pressure in some rifles - there is simply no short-cut to a safe and accurate 1000 yard .308 load, you have to do the work. I'll show you how, but I can't do it for you; if you can't do it, your best bet is to avoid the .308 at 1000 yards, your eyes (and your neighbor's) are worth more than another day at the range.

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.

To get started, enter your range altitude, temperature, bullet choice and a reasonable velocity. As an example, let's use my altitude of 1640 feet, ambient temperature of 65 degrees, the Berger 175 Long Range BT (Litz) and a muzzle velocity of 2830 which happens to be my standard load. The JBM results show that the remaining velocity at 1000 yards is 1.263 mach. The actual velocity number is interesting (1418 fps) but is actually irrelevant.  If we were to take this 2830 fps load to a range with an altitude of 50 feet above sea level, the remaining velocity at 1000 yards would be 1348 fps which is 1.201 mach - which number tells you more? That's right, the mach number tells you more, it tells you what you need to know!

Let's look at the numbers a different way; back at my local altitude of 1640 feet, how low can my muzzle velocity be with the 175 Berger LRBT and still stay at 1.2 mach at 1000 yards?  A minute or so trying different MV inputs gives me the answer - 2730 fps is the minimum muzzle velocity required to stay at 1.2 mach at 1000 yards with this bullet at my altitude and ambient temperature.  Therefore, my second magic number is 2730 fps.  These calculation need to be performed prior to the next step because that second magic number will be your focus in load development.  Remember, 2730 fps is MY magic number, not yours, you have to do the calculations to come up with yours - it depends on your bullet choice, the altitude of your range and the lowest temperature at which you expect to shoot.

Range Loading and Chronographing
The chronograph is an essential tool for developing your .308 long-range load - you really can't get by without it. Reloading at the range makes the process much more efficient and isn't particularly hard to do, I really recommend it at this stage of load development.

Get to the range with an adequate supply of sized brass, the primers, powder and bullets you intend to test, and get to work. Work up to your magic number velocity and record the charge weight and powder type and lot number. From there, if there are no adverse pressure signs, you can continue varying your charge weight as you seek accuracy, but remain vigilant for pressure signs! There's a hidden trap in this process - you can't develop a load that leaves you no safety margin because everything is subject to some change. It might be a hotter day than you predicted at the match so pressure increases, you might run out of your existing lot of powder and have to substitute another on short notice, who knows, many things can happen so don't take your loads right to the maximum.

The most important point to keep in mind is that additional velocity does very little to increase wind drift resistance. Find the load that gives you accuracy at safe pressures and is above your magic number for MV, after that, there isn't any increase in MV that will genuinely alter wind drift in a noticeable manner - but it sure can endanger you with excessive pressure!

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.

Coming Soon
In Part 4, we'll discuss how case capacity and seating depth can affect your loads and pressure.

All contents Copyright 2012 The Rifleman's Journal