Interpreting Load Data

Interpreting Load Data

by Morris Smith August 17, 2016

Many people regard published load data, whether it be from a book or a manufacturer’s website, to be a bible so to speak.  Reloaders both new and “experienced” sometimes incorrectly view it as the last word on which recipe to use when making a reloaded cartridge.

The first thing to realize is that the “recipes” and ranges provided in a reloading manual are based upon a specific test gun that may or may not closely match the firearm for which you are building the load. As an aside, I will mention that even if your buddy has what he thinks is an excellent load for his particular firearm, it probably will not be ideal for your firearm, even if you have the same make and model.  This is one of the biggest mistakes made in reloading.  The data provided in these manuals is thought to be a safe load for most firearms, but again, the assumption is that your firearm is similar to the test gun that the manufacturer used to come up with the data. Let’s look at an example:

I like to use the Hodgdon reloading website data as a starting point for most of my load workups www.hodgdonreloading.com. For this example we will use the data for my favorite .308 caliber load for my GA Precision GAP10 rifle. This .308 Winchester example uses the tried and true 175 grain Sierra Match King bullet and IMR 4064 powder.

Load data

Take a look at the highlighted portion of the graphic above. These are the specifics of their test barrel and the particular components they used to come up with this load data. You should also note that unless it specifically says differently, the data they provide is for a bolt action rifle. If your rifle, reloading components, or even ambient temperature is different, you will get different pressure and velocity outcomes than the ones that the load data provides. For example, I use Winchester and Lapua brass. Their example uses Winchester.  Now I will say that I needed to decrease my charge 2/10ths of a grain from my optimal Winchester brass load to get the same results with Lapua brass, so that by itself is an example of how your mileage may vary depending upon what brass you use. The example also calls for a 1:12 twist barrel, my barrel is 1:11.25. I have not been able to find Federal 210M primers for about 2 years so I use Winchester Large rifle primers. That’s another difference. My barrel is 22” not the 24” they used in their test. And finally, I am loading for a semiautomatic AR10, not a bolt action rifle. These types of differences can, and usually will, give you very different outcomes in terms of velocity and pressure from the ones that the load data provides, so the stated safe powder range may or may not be the safe range to use in your particular gun. The take away for all of this is that you should always do a proper load workup for the pistol or rifle ammo you are making. There are several ways to do a load workup, and I won’t get into them today, but they all start with one basic premise; start on the low side of the provided range and slowly step up to the high end of the range, while always looking for signs of high pressure in your particular gun. These are signs every reloader should know such as cratered primers, flat primer, blown primers, sticky bolts and extractor marks.

Load data 2

Some reloaders decide that they will just start making cartridges in the middle of the range, while others may decide to just pick a load at the high end and run with that, thinking that “hotter” is better. Here is the reason not to and it is the same one we mentioned earlier: chances are, your gun and chamber is different than the one used to develop the “safe” load range that the data provides.  Using my gun and load as an example, you can see from the data shown above that 45.6 grains of gunpowder will be compressed, but according to the data it should be safe.  It’s a good thing we know better. In actuality, I start seeing signs of pressure at 43 grains of powder in this particular gun, so if I jumped in at 45 grains thinking that it should be safe since it wasn’t quite the high end of the “safe range” I would be fine, when in reality I would be running dangerously high chamber pressures and risking my gun, my safety and the safety of those around me. The example we used in this article was based upon a .308 rifle round, but the same principles, warnings and cautions apply to handgun reloading as well.  Whether it be 9mm, .45 ACP or 500 S&W, never start in the middle of the range. Always start at the low end of the range and work up slowly toward the high side.

Load data is a starting point, not a one size-fits-all recipe!



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