1000 Yard .308 - Case Capacity and Other Problems
by Germán A. Salazar
by Germán A. Salazar
This article is the fourth in a series on loading the .308 for Palma or other forms of 1000 yard shooting. In the third part of the series, we stressed the importance of keeping the bullet at or above 1.2 mach at 1000 yards and gave you the tools needed for calculating the MV you will need in order to accomplish that goal. We called the resulting MV your "magic number." We now shift our attention to an examination of the .308 case's limitations and how to make effective use of its limited case capacity in order to reach the magic number safely.
All of this would be of little importance to us were it not for the simple fact that the rules of Palma shooting require using a .308 (or the even more inadequate .223). Therefore, we must find ways to minimize the adverse impact of the .308's limited case capacity.
Now that the obligatory History lesson is out of the way, let's turn to Physics. For those of you who slept through Physics class when you were in school, this is the time to pay attention. The first lesson to remember is that in a pressure vessel such as a cartridge case, pressure is inversely proportional to volume. In other words, for a given amount of fuel (powder) contained within the vessel (the case), pressure will increase as the volume of the vessel decreases. When the pressure level is 55,000 psi to 60,000 psi, very small changes in case volume will result in very large changes in pressure. Don't disregard the differences illustrated below simply because they appear to be numerically small - their effect on chamber pressure is quite significant.
The second physics lesson has to do with how chamber pressure and muzzle velocity are interrelated. Chamber pressure and muzzle velocity are directly proportional. Assuming the use of the same bullet, as you increase pressure, you will increase MV. However, not all forms of increasing chamber pressure will result in the same amount of MV increase. For instance, a hot primer might raise chamber pressure by 5,000 psi compared to a mild primer, but the increase in MV is relatively small compared to a 5,000 psi increase from additional powder. Why? The hot primer's explosion contributed to the peak pressure but because it generates very little gas volume compared to burning powder, the primer's extra pressure does very little to drive the bullet down the barrel to a higher MV. If you were to look at the pressure curves of two loads as just described (one with a hot primer and less powder, one with a mild primer and more powder) the peak pressure would be the same, but the load with the mild primer would hit peak pressure a little later in time and the down-slope of the pressure curve would be gentler as the additional gas volume from the powder charge keeps driving the bullet longer.
The third lesson: What if we want to compare the same bullet and powder type in two different cases? In order to reach a certain chamber pressure, you will use more powder in a higher volume case than in a lower volume case. Although peak pressure will be the same when the charges are properly adjusted to each case's volume,the higher powder charge in the light case will generate more gas which keeps working for you all the way down the barrel and ultimately will deliver a bit more MV at the same peak chamber pressure.
The fourth and final lesson is that heavier bullets work best with slow powders and slow powders require heavier charges in order to reach operating pressure. Slow powders also tend to have longer, thicker granules, making them pack less efficiently in the cartridge case, so a light, high volume case is highly desirable when shooting heavier bullets.
Getting the amount of powder necessary for our magic number MV into the .308 case, while maintaining safe pressure levels, is the objective. Our task is complicated by the fact that the variance in capacity of different types of .308 cases exceeds that found in any other cartridge by a wide margin. The failure to acknowledge that variations in case capacity have a significant effect on pressure has led many reloaders to very unfortunate results.
Let's take a look at a few types of .308 cases used (sometimes unsuccessfully) by long-range shooters. It isn't my intention to present all of the .308 cases that can be used, just a reasonable sample. The figures cited for case weight and case capacity are from a single case of each type, drawn from my inventory; obviously there is some variance within each lot and from lot to lot. However, I checked twenty samples of each type for case weight and picked the sample case from the middle of the range. Case capacity was measured on cases in full-length sized condition. Please don't take these weights and volumes as being any sort of an absolute; check your own cases and make your own comparisons. My purpose is to show you the how and why of things, not to do your work for you.
WCC 58 - For many years, this was the Holy Grail of .308 long range shooters, Western's 1958 match brass has enough capacity to drive the 190 Sierra to its magic number MV with IMR 4064. The development of high BC bullets and powders with greater density than IMR 4064, eliminated the WCC 58 case's unique status for long-range .308 shooting.
Case weight - 147.1 gr.
Case weight - 156.25 gr.
Case capacity - (water to top of neck) - 56.75 gr.
Case weight - 157.40 gr.
Case capacity - (water to top of neck) - 55.80 gr.
Case weight - 177.45 gr.
Case capacity - (water to top of neck) - 53.45 gr.
When is a .308 Not a .308?
Although the case itself is the primary determinant of powder capacity, the chamber dimensions also play a role. Notwithstanding many shooters' somewhat misguided perception that a "tight"chamber is an accuracy enhancer, the opposite condition, a somewhat large chamber, is more likely to be a benefit to the match shooter. A case fired in a chamber that is close to the maximum dimensions standardized by SAAMI can hold about 2 gr. more powder than one fired in a so-called "tight" chamber. Of course, unless the reloader has a special sizing die made to match that fat chamber, there will be no benefit realized and given the expense of such a die, this is not a very popular approach, although it is not unheard of. As with the super-light brass, the big chamber is less interesting now that we have a broader range of choices than just the 190 Sierra and IMR 4064, a good, traditional combination, but one that requires all the case volume possible.
A frequently seen chamber variant is the long throat. An increase in throat length of 0.100" will yield approximately a 2 gr. increase in case capacity; similar to what the fat chamber will do. The relatively short neck of the .308 case makes a long throat an important consideration when bullets in the 175 gr. and higher category are to be used. However, unlike cartridges with a longer case neck such as the .30-06 which can easily handle bullets across a broad weight range with a standard throat, the .308 will be limited to a relatively narrow weight range of bullets with a given throat length. This isn't to say that you can't physically load and fire any bullet weight in a standard .308 throat; but simply that you will may not be able to effectively load them for 1000 yard shooting.
So when is a .308 not a .308? Any time you have a chamber with dimensions that differ from SAAMI and that includes most match chambers. That is why you need to be extra cautious when listening to load data from others. Most .308 match chambers are actually a semi-wildcat and load data from one may well be dangerous in another - they aren't all alike!
Throat length is another important consideration for the long-range .308; seating depth will be determined by the throating and it will affect chamber pressure. As we noted earlier, pressure is increased when case volume is decreased; therefore, throat length should be selected with a close eye on the bullet weight you intend to use. Heavier bullets present special problems as the powders most suited to those loads tend to be bulkier and require more case volume while the heavier, longer bullet will soak up case volume unless a special throat length is used.
Let's look at some photos of seating depth variations using the Sierra 155 gr. and 190 gr. bullets with two different throat lengths. The two chambers used for the comparison are the Palma 95 chamber which is fairly standard for 155 gr. bullet users and a chamber with a longer throat, more suited to 190 gr. bullets. The chamber I presently use is a bit shorter than the long-throat chamber I show here as this one is a bit too long for the 175 gr. Berger which I use most of the time.