Why does reloading data change between manuals?

Why does reloading data change between manuals?
April 27, 2020 Sierra Bullets

Why does reloading data change between manuals?

By Sierra Bullets Tech Team

Here’s a question we get regularly at the Sierra Tech Line, and we wanted to share it with everyone! Send us your questions, and we might feature it in another article.


“I just purchased a sixth edition Sierra manual and was comparing load data from the fifth, specifically for the 270 Winchester. I noticed that the max charge for many of the powders was dropped, resulting in the max velocity dropping. I saw where there was a drop in the barrel length, which drops the velocity; but I was wondering why the max charges were dropped for some powders.”


A reloading manual is a collection of information of results obtained by using a specific set of components in one specific firearm. Any changes in that set of specifics can, and normally will, cause different results. A quality Reloading Manual will try to spell out as many specifics as possible to make the user aware that there are differences between what was done in the data collection process and what takes place at their reloading bench. This is all to bring a high awareness to safety. When normal pressures can be in the realm of 60,000 to 65,000 psi, anything that changes can cause those same normal pressures to become quite excessive and very possibly catastrophic.

When new manuals are created, you always start off with a new barrel/firearm. Then you may have a different brand of brass, primers, etc. You certainly will be using powder from different lot numbers. All of these changes in components and the firearm are going to give different results. The only way you can have the same results would be to run the exact same test with no change in any components.

We as reloaders need to realize that Reloading Manuals are there to give us a safe starting load. Everything else is a result of what took place in the test.

Now, when working up loads for our own firearms, we again need to realize that we are working with a completely different set of components. This is why it is very important to start at the low end of the load data tables and work loads up carefully. With all of the changes, excessive pressures can happen at any point.

Differences in velocity are very subject to the changes in components as well. The ammunition can be from the same box or from the same batch that you loaded, but when fired in different firearms, you can certainly expect the pressures/velocities will change. Velocity change can be rather dramatic. It is not unheard of to experience as much as 100 fps to 200 fps change just by the ammunition being used in a different firearm.

So, please be careful. Reloading is a very rewarding experience. We want you to enjoy and share your experience with family and friends. Safety is the utmost concern.

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