Modern Intermediate Calibers, Interlude: How, Why, and What for?

    5.56mm (left) alongside some of its competitors.

    At this point we’ve looked at the data for seven intermediate calibers currently on the market, each of which is – one way or another – influencing the discussion around the question of what next generation military rifle caliber will be. Those rounds were: The 5.56mm NATO, the 7.62x39mm Soviet, the 6.8x43mm SPC, the 6.5x38mm Grendel, the .300 AAC Blackout, the 7.62×40 WT, and the .25-45 Sharps. Initially, I intended for this series to be limited to just these seven, and I picked them as a cross section of different concepts and schools of thought regarding the intermediate caliber problem. Now, however, I have decided to expand this discussion to other calibers, including the latest developments outside the West. I figure I’ll just handle the extra clunk this introduces by breaking up the final comparative discussion into segments, that way all the data is presented clearly, and there are more posts for my readers to chew on. Everyone is happy.

    Speaking of data, I’d like to talk a little bit in this quick intermission about how I created the graphs in each post, what my methodology was, and how I determined what my inputs and outputs needed to be. As stated in each post, I use the excellent ballistic calculator available for free in your browser from JBM Ballistics. I’ve mentioned this before, but as far as calculators go, JBM’s is the best one that is readily available and which doesn’t take a degree to use. It allows both the most control over the inputs (by far) of any calculator I have used, and it gives the most accurate results. It properly accounts for a bullet’s transition from the supersonic to transonic and then subsonic regime, which many calculators do not do. However, it is not perfect! There are many complex things going on in a bullet’s flight that a browser calculator simply cannot account for, so I would tell my readers to accept the data I provide as a general guideline, not the final word on how these calibers actually perform! That can only be determined through empirical testing, which is far too expensive for Your Humble Blogger to do at all, much less properly.

    With that out of the way, we can talk about what inputs I actually used for the JBM calculator. I use the stock inputs for the JBM – Trajectory calculator, except for the following:

    • Distance to Chronograph – set to “0” instead of “10”
    • Range Increment – set to “10” instead of “100”
    • Zero Range – set to “25” instead of “100”
    • “Energy Column” Formula – set o “Energy (Joules)”
    • Check the “Ranges in Meters” box

    For each individual round, I enter the bullet’s weight in grains, its caliber, its muzzle velocity from the barrel length I am using, and the sight height – which for most was a value of 2.6″, representing an AR-15 or AR-10.

    Although different calibers will have different optimum zeroing ranges to give the flattest trajectory, since my goal was to compare the different calibers’ performance, I kept a constant zero distance value of 25 meters. This is a common zeroing distance for military rifles, and allows an even comparison of the shape of the trajectories of the different rounds being considered. Likewise, even though wind speed and angle will vary in real life, I kept a constant value of 10 miles per hour moving at a 90 degree angle across the sight line.

    Many American readers will likely raise an eyebrow at the use of the Joule as a unit of energy, instead of the Imperial foot-pound. I am not really a proponent of metrication, but I find the Joule much easier to work with than the foot-pound. It is in fact just as easy to convert Imperial values to metric and then from them calculate Joules than it is to use Imperial values to calculate foot-pounds. Mixed units are just a given for military small arms ballistics discussions, so I don’t worry too much about mixing the systems at this point.

    Readers who have been following my Ballistics 101 series of posts know that it’s important to use the correct drag model (ballistic coefficient model) for a given bullet. So, rather than use a constant drag model (e.g., G7), I am using the models appropriate for the projectiles being considered – in other words, the model a given bullet most closely resembles physically. In practice, this basically means G7 for boattailed bullets and G8 for flat-based ones, as those are the two shapes in common use for military rifle projectiles today.

    As for the outputs, I have them saved in spreadsheet files in a folder on my computer. I plan to release the raws to my readership at some point, although since at the moment I am away from home traveling I cannot do it now. Whatever I decide to do, whether that’s release them all at once in a downloadable folder in the final post or add each caliber’s data as a batch of files to their respective posts, by the time I am finished, you will all be able to see the actual data I used.

    I’ve been long-winded so far, so I am going to wrap this up, but the last subject I wanted to cover here regards the reasons I have decided to do this series and the direction I would eventually like to take it, and how that fits in the wider discussion about military calibers. With these posts in general, I wanted to create an even, easily-referenced foundation of data for each of these calibers that could be referenced by myself and others when discussing them online. While data for almost all of these calibers exists and can be found on the Internet, one ballistic chart for one caliber may not be directly comparable to another for a different caliber, because it may not use the same units, the same firing platforms, or the correct drag model. My goal then is to give my readers a very clear idea of what each of these calibers can – in theory, at least – do compared with each other, and also to give my readership a solid foundation of reference material for the discussion when it evolves beyond just current military caliber hopefuls. That is why it is very important for me to not only get the data as right as possible, but also to make my methods and procedures transparent.

    Anyway, we’re not done with Modern Intermediate Calibers yet, so stay tuned for next time when we tackle the Soviet 5.45x39mm!

    Nathaniel F

    Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. He can be reached via email at [email protected]