The bullet jump controversy: Specials in Magnum chambers.

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I got an email recently from a reader who asked about .38 Special accuracy when fired in a .357-length chamber. There is, as he noted, a lot of speculation on the topic: some saying they’re less accurate, some saying it doesn’t matter, and others saying that there is no way we’ll ever know for sure.

I’m not at all convinced about that last one, but the first two opinions are both correct – under some circumstances. Some years ago I experimented with this, and what I found comes under the heading of “it depends.”

The concern is that the unrestrained jump of the bullet from the shorter Special case causes instability and thus inaccuracy. A Magnum chamber is longer from the rim seat (the area at the back of the cylinder where the rim makes contact) to the chamber throat (the narrow area at the front of the cylinder that guides the projectile into the barrel.) When a Special cartridge is inserted into the longer chamber, the bullet has to travel a distance (called “jump”) before it reaches the narrower throat. In this distance, it’s thought, the bullet can yaw slightly.

I’ve done up this little graphic (greatly exaggerated and not to scale) to illustrate the situation:

Notice the area between the bullet and where the chamber mouth begins – that’s the freebore area where the bullet’s travel is unrestrained and, according to theory, starts to wobble to the detriment of accuracy.

A number of years back I did some experimenting by loading the same bullets in .357 Magnum and .38 Special cases, and adjusting the velocity so they matched. I found that sometimes the Specials did show a loss of accuracy, while at other times they didn’t. (I had one case where accuracy with Specials actually improved.) Why the variance? If the bullet jump is responsible for accuracy degradation it should be consistent, and it certainly wasn’t.

The answer is that the freebore is only part of the equation.

As I’ve written before, one of the most important contributors to accuracy in a revolver (and the MOST important when shooting lead bullets) is the chamber throat. Assuming that the bore diameter is correct, a throat which fits the bullet precisely will deliver greater accuracy than one which is oversized (or undersized to a great degree.)

If the throat is larger than the bullet diameter – say, .001″ or better – accuracy drops off. If the throat and bullet match, accuracy will generally be at its best. If the throat is slightly (up to .001″) smaller than bullet diameter, jacketed bullets will usually show a falloff in accuracy but lead bullets usually won’t, at least not to the same degree. (More testing is needed in this area, however, as I don’t have enough data points with smaller-than-bullet throats to reach a definite conclusion.)

When the throat diameter was the same as the bullet diameter, there was generally little to no difference in accuracy between the long and short. When the throat diameter was larger, however, the Specials were usually less accurate than the longer cases. Someone doing the same experiment but not taking into account throat/bullet diameter matching would probably reach different conclusions, which I believe is the source of the varying opinions and the reader’s confusion.

More experimentation should be done, however, to eliminate other variables such as the angle of the transition between chamber and throat and any surface irregularities in that area.

I also would expect the same dynamics to apply to larger calibers such as the .44 Magnum and Special, though I have no experimental data to prove my supposition.

-=[ Grant ]=-

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About the Author:

Grant Cunningham is a renowned author and teacher in the fields of self defense, defensive shooting education and personal safety. He’s written several popular books on handguns and defensive shooting, including "The Book of the Revolver", "Shooter’s Guide To Handguns", "Defensive Revolver Fundamentals", "Defensive Pistol Fundamentals", and "Practice Strategies for Defensive Shooting" (Fall 2015.) Grant has also written articles on shooting, self defense, training and teaching for many magazines and shooting websites, including Concealed Carry Magazine, Gun Digest Magazine, the Association of Defensive Shooting Instructors ADSI) and the popular Personal Defense Network training website. He’s produced a DVD in the National Rifle Association’s Personal Firearm Defense series titled "Defensive Revolver Fundamentals" and teaches defensive shooting and personal safety courses all over the United States.
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