Why Does Reloading Data Vary?

 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.

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6 Responses to Why Does Reloading Data Vary?

  1. Dennis Cattanach says:

    It is all statistics. If my chrony comes in within a 100 fps on the 300 weatherby or 2506, I am happy. Us reloaders add our own set of .tolerances. I am surprised that the reloading data among the manfactures agree as close as they do. Somebody is doing something very correctly.


  2. Brittius says:

    Reblogged this on .


  3. Ray Grigsby says:

    At one time I owned two Ruger rifles in 300 Win Mag. One was a Model 77 and the other a #1V.
    I had worked up a nice accurate load for the 77 that was below the listed “max”. Just to see how it shot I decided to try the load in the #1V. The first round I could tell was hot. When I managed to open the action it did actually extract but the primer fell out of the case head😱 Being young and dumb at the time and with no apparent ill effect I proceeded to try a couple more with the same result just to make sure😁
    Bottom line start in the low middle range of the suggested loading data and work up slowly. If you start to see pressure signs as you shoot your “ladder” stop and disassemble those hot rounds. If you find a load the shoots tight groups settle on it😁


  4. Al says:

    well said,


  5. Please note, the essay presented is completely accurate and correct. All artifacts (man made objects or devices) are subject to ‘tolerances’.

    Not mentioned as such are the firearms themselves. No two firearms – same type, barrel length and caliber – are exactly the same. Consequently, they will vary on ‘most accurate load’ and ‘fastest load’ forever. (Let along arms of the same caliber but different makers.)

    And the same rifle or handgun will vary with the same load depending on the temperature and light conditions.

    And – let’s face it – sometimes the shooter isn’t having such a great day. Can’t blame that on the bullet or powder or primers.


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