Written by Sierra Bullets Ballistic Technician Paul Box
Everyday we get calls on our 800 line about why does data vary from one manual to another? It’s an honest question and we’re going to take a deeper look into why this happens. The short answer is tolerances, but let’s see what this means.
Everything in this business has a tolerance to stay within. Let’s look at powders first and use IMR-4064 as an example. When the powder manufacturers make a lot of powder, it will be tested to make sure the burn rate is right. Again, just for example let’s say it must fall between one and ten on a scale. The first lot comes in at eight and the second lot at three. They are both within spec, but the second lot is slightly faster.
Brass will be the same way. There will be differences in case weight from one maker to another and this will influence internal capacity.
Primers will have a difference in both flame duration and flame temperature. Again, this will influence pressures also.
Looking at rifles, we see the same thing. Chambers can vary slightly and still be in SAMMI specs. Throat lengths will not only vary from one rifle maker to another, but even the same rifle manufacture will have a difference depending on the reamer that was used. It’s not unusual to see one rifle in .308 vary by .035” from another of the same maker. This also influences max loads.
So what does all of this mean? If one manufacturer shoots data and just by sheer chance has some of their components and their rifle on the upper end of specs, then another manufacture by sheer chance has theirs on the minimum side, we can see a big difference on a max load between different manufactures. On a big cartridge like a 300 Win mag and a slow powder like RL-22 I’ve seen 2.0 grs. difference on a max load between the different manufacturers. So, who is right? Actually they both are. The data from each manufacturer is correct for the components they are using in their rifle.
For many years we have all read in reloading manuals as well as countless magazine articles about the importance of starting low and working up. This is sound advice. Remember, you can always go up.