Why Ballisticians Get Grey
by Dave Andrews

(Editor's Note: This article was originally printed in the Speer Reloading Manual #9 published in 1974.  When planning the #12 Manual, it was my intention to write a new section discussing velocity variation among different firearms chambered for the same cartridge.  Then I re-read Dave's article and decided that twenty years hasn't aged this information at all.  Dave's insightful essay holds as true today as it did in 1974, and we still get requests for reprints.  Thanks to writer Dick Metcalf for his gentle urging to reprint this.) [J. Allan Jones, Editor, Speer Reloading Manual #12 - Click Here for the Speer website]

"Dear Ballistician:  Your reloading manual is all wrong!  You say on page 713 that 11.2 grains of Super-Duper powder will push the 125 grain bullet at 1468 fps.  My barrel length is the same as yours, but when I tried this load and had my friend chronograph it the velocity was only 1411 fps.  Why are you so far off?"

Letters like this imaginary one are all too common.  In an effort to pinpoint one reason for such velocity differences, the Speer Ballistic Laboratory selected three lots of .357 Magnum ammunition in different bullet weights.  These particular lots of ammunition were selected because of their uniformity, not because of high velocity.  The ammunition was fired in all of the .357 Magnum guns available to the lab at the time.

The different handguns were all tested in same manner with the gun muzzle elevated and then gently lowered to the horizontal for each shot.  Every effort was made to make the results as accurate as possible.

The table shows the average velocities of the three different bullet weights in each of the guns tested.  Note that in the standard 10" test barrel, made to tight ammunition industry specifications, the extreme variation (EV) in the velocities ranged from 48 fps for the 125 grain hollow point bullet, to 38 fps for the 158 grain soft point bullet.  Using the 6" barreled revolvers as an example, the EV between all 125 grain bullets fired in all the 6" barrels was 376 fps, almost 8 times the EV in the test barrel.  The EV for all 6" barreled revolvers with the 140 grain hollow point ammunition as 275 fps, over 10 times the EV in the test barrel.  The 158 grain soft point ammunition showed an EV of 282 fps, almost seven times the test barrel EV of 38 fps.

These large variations are due partly to the relatively very small differences in chamber, bore, forcing cone rifling, and barrel-cylinder gap dimensions and in the finish or smoothness of these interior surfaces.  Chambers will vary minutely even though cut with the same reamer, as will all other machined surfaces.  It is virtually impossible to manufacture two of some machined metal item, even as simple as a revolver, with all dimensions and finishes exactly the same.  When hundreds of thousands of .357 Magnum revolvers have been made by thousands of different people, in different factories, with different materials and tooling, it cannot be surprising that there are differences between guns.

These minor differences between guns cause some of the differences in ballistic measurements.  Additional variations, due to differences between different makes or lots of bullets, powder, primers and cases, powder charges, loading dies, loading techniques and chronographs complicate the problem.  Many times these small differences tend to cancel each other, but when everything goes one way, the resulting variation may be relatively large.

These velocity tests are not presented with any idea of claiming that one particular brand or model of revolver is superior to another.  A repeat of the test with different ammunition might well reverse the relative standings shown here.  The point we want to make is that even with the very best quality ammunition available, there will always be velocity variations when the ammunition is fired in a different gun.

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Copyright 1994, 1998, Blount Inc. All rights reserved. Reprinted with permission.