Ballistics by the inch

The illusion of precision.

Got an email which is another aspect of the problem I wrote about recently. The author was asking that we get more fine-grained in our data, by making measurements of barrel lengths by one-eighth and one-quarter inch increments. Here’s a couple of relevant excerpts:

what more is really needed, is barrel lengths between 1-7/8 and 4-1/2″.
because of the proliferation of CCW and pocket pistols, and unresolved
questions about short barrel lengths that go all over between 2 and 3.75″,
and snubby revolvers that may be even shorter.

* * *

with that amount of precision, not only would you have data covering all
lengths of short barrels, but you could fabricate mathematical curves that
would predict velocities for any possible barrel length, metric or
otherwise, given the particular ammo.

 

It’s not an unreasonable thought, on the surface. Our data clearly shows that the largest gains in bullet velocity always come in length increases of very short barrels for all cartridges/calibers. So why not document the changes between, say, a 4.48″ barrel and a 4.01″ one? That’s the actual difference between a Glock 17 and a Glock 19, both very popular guns which are in 9mm. Or between a S&W Model 60 with a 2.125″ barrel and a S&W Model 360PD with a 1.875″ barrel?

Ideally, it’d be great to know whether that half or quarter inch difference was really worth it, when taking into consideration all the other factors in choosing a personal defense handgun.

The problem is that there are just too many different variables which factor into trying to get really reliable information on that scale.

Oh, if we wanted to, we could do these kinds of tests, and come up with some precise numbers, and publish those numbers. But it would be the illusion of precision, not actually useful data. That’s because of the limits of what we can accurately measure and trust, as well as the normal variations which occur in the manufacturing process … of the guns tested; of the ammunition used; of the chronograph doing the measurements; even, yes, changes in ambient temperature and barometric pressure.

That’s because while modern manufacturing is generally very, very good, nothing is perfect. Changes in tolerance in making barrels can lead to variation from one gun to the next. Changes in tolerance in measuring the amount of gunpowder which goes into each cartridge (as well as how tight the crimp is, or even tweaks in making the gunpowder itself) mean that no two batches of ammunition are exactly alike. And variations in making chronographs — from the sensors used, to slight differences in positioning, to glitches in the software which operate them — mean that your chronograph and mine might not agree on even the velocity of a bullet they both measure.

All of those little variations add up. Sometimes they will compound a problem in measuring. Sometimes they will cancel one another out. But there’s no way to know which it is.

This is why we’ve always said to consider our data as being indicative, not definitive. Use it to get a general idea of where your given choice of firearm will perform in terms of bullet velocity. Take a look at general performance you can expect from a brand or line of ammunition. Compare how this or that particular cartridge/caliber does versus another one you are considering.

But keep in mind that there’s no one perfect combination. You’re always going to be trading off a bunch of different factors in choosing a self-defense tool.

And never, ever forget that what matters most — FAR AND ABOVE your choice of gun or ammunition — is whether or not you can use your firearm accurately and reliably when you need to. Practice and training matters much more than whether or not you get an extra 25, or 100, or even 500 fps velocity out of whatever bullet is traveling downrange. Because if you can’t reliably hit your target under stress, no amount of muzzle energy is going to do you a damn bit of good.

 

Jim Downey

If you want more information about how accuracy and precision can be problematic, this Wikipedia entry is a good place to start.

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September 6, 2015 - Posted by | .357 Magnum, .38 Special, 9mm Luger (9x19), Data, Discussion., General Procedures | , , , , , , , , , , , , , , , ,

2 Comments »

  1. Search YouTube for “Ammo Quest 9mm” for a long series of videos done by “ShootingTheBull410”. He tested many different brands, bullet weights and pressures of 9mm ammo from a 9mm pistol with a _3 inch barrel_, accurately measuring bullet speed in feet per second and well standardized ballistic gel penetration tests with and without 4 layers of denim as per official FBI standards. Very good objective testing to see what he got with his particular 3 inch barrel little handgun. As you say in your article above, these are good _indicative_ tests of how different 9mm offerings in 115 grain or 124 grain or etc bullet weights in standard pressure or +p and pressures and so forth actually do.

    When you search YouTube you will find he did a whole series of tests with many different 9mm ammo brands so what you are using in your compact or sub-compact little 9mm concealed carry pistol is likely to be in the list somewhere. Nice to SEE what the different ammo brands and types do when fired from 3 inch barrel. My particular pistol has a 3.2″ barrel so I found these videos very interesting. Almost all such tests found on YouTube are usually done with 4 inch or longer barrels so these videos are great viewing for concealed carriers who use 9mm with shorter barrels. I have no association with “ShootingTheBull410″ other than I found his videos and liked his professional / standardized testing methods.

    Worth watching for _indicative_ results using a 3” barrel objectively testing many different ammo brands and loadings so you can comapre the results for yourself. Certainly can’t hurt to look them over in any case. 🙂

    Comment by PA John | February 5, 2016 | Reply

  2. Missed this one, but I can most confidently predict that such a fine-toothed approach to data collection would yield a band of data that almost perfectly brackets Jim’s already fine results. Interpolation of the listed data Jim gives us in every test is more than adequate to get one on the right path, THEN personal testing of custom hand loads, and tight control of chrony and weather data can hone in on the proper setups. You can get a nearly perfectly optimized load for your own choice of carry weapon with less headache than it would take for BBTI to do .125″ incremental testing 😉

    Comment by underground12x8 | June 5, 2016 | Reply


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