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Caliber Confusion: Why are .38 Specials .36 Caliber?

By Rob Behr
A couple of years ago, a lawyer I respect and one of his co-counsels offered to take me to lunch in exchange for answers regarding a firearms-related case they were defending.  Being hungry, and typically reduced to offering firearms related prattle for free, I leaped at the chance.

A .357 Magnum alongside a .38 Special.

A bullet had been recovered from a crime scene and been duly identified by Montana’s crime lab as .357 caliber.  Always looking for an inconsistency that might carry some weight with a jury, the two lawyers told me that the bullet was supposed to have come from a .38 Special revolver.  They both saw this as a substantial inconsistency which might tend to exonerate their client.  For all of you who plan to work as a forensic expert, I am now going to offer some sound advice:  Make them buy lunch before answering questions like this.  Otherwise you may be left to pick up the tab.

A bullet slugged through a .38 Special barrel on the left is compared to an unused bullet on the right.

When I told them that the .38 Special used .357 caliber bullets, the co-counsel told me, swelling with self-confidence, that he had been a JAG lawyer in the military and that he knew you could not fire .357 Magnums in a .38 Special.  He had me there.  He also had a bad case of Caliber Confusion.

Of course the crime lab had not told him that a .357 Magnum handgun had been used, although it certainly could have been.  They had simply provided the diameter of the bullet that was recovered at the scene.  What was telling in the conversation was that the lawyer knew you could not fire a .38 Special revolver using .357 Magnums.  He was also aware that you can fire .38 Specials in a .357 Magnum handgun.  He was simply unable to make the obvious leap that both cartridges are the same caliber with different chamber dimensions.  The use of caliber to describe the loaded cartridge is so common that they are often used synonymously.  It can lead to problems that go well beyond simple confusion and is at the root of some pretty spectacular firearm failures.  Bullets have calibers, barrels have calibers but a chamber, the engine of a firearm, is made to match a specific cartridge.

The slugged bullet measured in at .357″

A cartridge consists of a bullet, propellant, primer and the case itself.  From the viewpoint of a ballistician, the internal ballistic is determined by dynamic forces produced within the chamber.  Once the bullet has left the barrel, the only hallmark of the internal forces that drove it is the velocity.  It would be very difficult for a ballistician to tell you unequivocally what cartridge propelled the bullet down range unless it was matched to a specific firearm and even then, in the instance of a handgun chambered in .357 Magnum, it would be difficult to tell if it had been fired using a .38 Special cartridge.  Typically the fired casing would have to be recovered to offer a real insight into which cartridge had been used.

In this specific case, the evidence locker held a .38 Special revolver and a .357 caliber bullet recovered from the crime scene. The bullet had been linked via rifling engraving marks to that handgun.  From a ballistics standpoint, the case was closed and lunch was finished.  There would have been no free lunch at all if the lawyer had been more familiar with cartridge nomenclature and the weird history of how cartridges get their names.

A 38/44 Smith and Wesson Outdoorsman.

A Brief History of the .357 Smith and Wesson Magnum
On one level, it is easy to explain the history of the .38 Special and the role it played in the creation of the .357 Magnum.  Despite the difference in lengths, the .38 Special is truly the parent case of the .357 Magnum.  High pressure .38 Special loadings, explored by several influential wildcatters gave way to specialized commercial loadings for handguns like the 38/44 Smith and Wesson Heavy Duty.  The .38/44 designation indicated that this pistol, using a .44 Special frame, was intended for use with high pressure .38 Specials.  These handguns bridged the gap between the .38 Special and the .357 Magnums that would be released a few years later in 1934.

When released, the commercial .357 Magnum cartridges were designed with a slightly longer case to prevent chambering in .38 Special revolvers.  Since both cartridges share the same caliber, case diameter and headspace, .38 Specials will fit and fire safely in .357 Magnums.  To be fair to our dining partner, what really confused him was that one caliber would seem to be .380″ in diameter and the other .357″.  If that wasn’t the case, why would they have such different names?  The answer lies in a popularity of .36 caliber cap-and-ball revolvers, the beginnings of the metallic cartridge age and finally the invention of internally lubricated bullets.

A Colt Paterson Revolver.

Why .38 revolvers are .36 Caliber
The Colt Paterson revolver was the first commercial revolver, hitting the scene in early 1836.  It was made in .28, .31 and .36 caliber versions.  It failed, but a later .36 caliber Colt, the 1851 Navy, became a world-standard revolver.  The 1851 and its variants were still in service well into the metallic cartridge age.  Unlike the current American system that assigns caliber by measuring from bottom of the groove to the bottom of the opposite groove, these calibers were assigned by bore diameter.  This measurement, taken from the top of the land to the top of the opposite land varies between .371 to about .365 in the 1851 Colts.  Bottom of the groove dimensions were quite a large running around .379.  Modern Uberti replicas use a bore measuring .360 and grooves that measure .384.  By our current system, the bullet diameter would be considered to be in the neighborhood of .380″.

Bottom of Groove to Bottom of Groove measurement now typically establishes caliber in the United States.

Measuring from the tops of the lands establishes bore diameter.


1851 Colt Richards Mason Conversion. Many were made in .38 Rimfire.

When Colt finally began to offer metallic cartridge revolvers — after the lapse of the Rollin White patent on bored through, rear loading cylinders — converting their well-loved .36 caliber revolvers to metallic cartridges was a priority.  The problem was that a cartridge occupies some of the cylinder space that had been filled only by the powder and bullet in the cap-and-ball revolver.  The bullet itself could not be fitted completely inside the case, because it would be severely undersized to the bore.  The answer to this problem was a heeled bullet.

This .38 Short uses a heel-type bullet and external lubrication. Note that the exposed bullet is the same diameter as the case.

Heeled bullets use a major diameter that matches the outside diameter of case, with an inner portion, the heel, that is small enough to fit inside the cartridge.  These bullets were about .380″ in diameter, conforming to the available space within the modified cylinder and were held in place by a heel of about .360″ that could be inserted and crimped inside the case.  Once fired the bullet performed like the cap-and-ball projectile, swaging into the rifling once it left the cylinder.  These cartridges, the .38 Short and .38 Long were measured by the external bullet diameter and naming them .38s made obvious sense.

As cartridges evolved, heeled bullet became less popular because they required external lubrication that attracted grime and dust.  With the birth of internally lubricated cartridges, beginning with the .44 Russian, the die was cast and heeled bullets were quickly considered obsolete.  Now the .22 Long Rifle and its family are the only commonly encountered cartridges of this type.  The rest are the domain of cartridge collectors.

This later .38 Short Colt featured internal lubrication and a smaller diameter bullet.

In order to make the new method of lubrication work, bullets needed to have a bearing surface that would both come in contact with the bore and remain protected within the cartridge.  The easiest solution was to make the whole bullet the same diameter as the bullet heel had once been.  This protected the lubricant from outside contaminants, as it was seated deeply inside the cartridge.  Now, however, the bores of all of these converted revolvers were much too large to swage the new .357″-.364″diameter bullets.  In order to fit these new bullets, the barrel dimensions had to be reduced.  They took on the now common .355-357″ groove depth and .346-351 bore diameter that are the recognized barrel dimensions for the .38 Special and .357 Magnum cartridges along with a sea of later mid-bore arrivals like the 9mm Parabellum and .380 ACP.  By that time, precedent had been set and many cartridges had adopted .38 as part of their name.

There is such a thing as a free lunch
I’m not saying this history lesson is a useful thing, unless you enjoy the evolution of firearms and cartridges, but it might be worth a free lunch.


What caliber is this bullet? It came from a .357 barrel. What cartridge fired it? Any number of medium bore cartridges. Without more evidence, there can be no substantive conclusion.