February 06, 2023
By Allan Jones
As I look through notes and papers from both my careers (Dallas County Institute of Forensic Sciences Crime Laboratory and Speer), I see more little quirks of the art and science of ballistics. Here are a few that came up over my 35 years of employment as a “ballistics guy.”
Cast Versus Jacketed Bullets and Pressures
There is a common and reasonably accurate prediction that, all other things being equal, cast lead bullets will produce less pressure than jacketed bullets of the same weight. Speer tested lead handgun bullets cast from RCBS molds to support its line of casting equipment. An anomaly in testing often becomes the “guest of honor” at breakroom discussions, and my tech found one.
RCBS mold No. 40-180-FN for the .38-40 Winchester was a great candidate for the then-new .40 S&W. Diameter and weight were spot on, and the nose ogive eliminated rifling clearance issues. We tested it with the same component lots and pressure barrel we used for 180-grain JHPs.
My tech returned from the range house with the data and said, “I don’t know what you’ll see when you crunch the numbers, but something is odd in the raw numbers.” In the final analysis, the lead bullet averaged higher pressures with the same charge weights than its jacketed counterpart. That was not expected.
Finally, it snapped into place. The cast bullet’s generous grease groove made it longer, which had previously been a non-issue. Yet .40 S&W was the shortest and least capacious cartridge in which we’d tested cast bullets at that time. We changed the prediction that cast lead bullets usually will produce less pressure than jacketed bullets of the same weight, adding “except in small-capacity cases.”
Primer Pocket Depth
Although Small Rifle and Small Pistol primer pockets share the same depth spec, large primer pockets do not. Per industry dimensional guidelines, the max depth of a Large Pistol primer pocket is 0.123 inch (3.12mm). For a Large Rifle pocket, the max depth is 0.132 inch (3.35mm).
The problem begins when people try to eliminate primer flowback in high-pressure handgun loads by using Large Rifle primers. The logic is not flawed—rifle primers are engineered to resist more pressure—but the actual action is flawed and dangerous.
The min/max height dimensions for a Large Rifle primer are 0.123 inch/0.133 inch. The min/max Large Pistol primer pocket is 0.117 inch/0.123 inch. In the best-case scenario, that would put the rifle primer flush with the case head, when proper safe practice is to seat the primer below flush with the case head. In a revolver a high primer can, at best, stop cylinder rotation and, at worst, under massive recoil, cause a sympathetic detonation of a cartridge not directly behind the barrel.
Pressure and Expanding Bullets
While I was visiting the FBI Firearms Training Center in the early 1990s, the senior ballistic tech approached me privately about an odd result in gelatin block testing. They had a collection of wound cavity data for the “10mm Lite,” driving a 180-grain JHP at about 980 fps. When the .40 S&W appeared, the same brand of bullet at the same velocity produced a smaller temporary cavity by a consistent and non-trivial amount, roughly 30 percent smaller (as I recall).
The tech had been through every subsystem with a fine-tooth comb, tweaked loads, and had enough test runs for statistical validity. He was really good at this, yet he was stumped. I would have been, too, but for prototype testing we’d done earlier at Speer.
I looked over the data and thought “pressure,” remembering the 10mm Lite testing at Speer. The 10mm FBI load posted a comfortable 24,000 psi; the smaller .40 S&W case needed 31,000 to 34,000 psi to make the same velocity. So?
In developing what grew into the Gold Dot line, Speer had fired prototypes of 400-180 plated HPs into non-deforming bullet-recovery media to compare behavior of bullets with dead-soft lead cores to those with lead-alloy cores.
Elmer Keith called the phenomenon “slugging-up,” where the bullet is stressed in-bore under pressure. Slugging-up increases the length of a bullet’s bearing surface at the expense of the nose length and, in hollowpoint bullets, can reduce the volume of the HP cavity. Slugging with soft cores was minimal at 24,000 psi but quite pronounced at 32,000+ psi. The FBI’s .40 S&W bullets were not the same shape when they hit the gelatin as the less-deformed 10mm Lite bullets. They had in-bore squeezing of the bearing surface, which lengthened and hardened it. Encroachment of the bearing surface up the nose limits how far a bullet can open on impact. The higher-pressure .40 S&W loads showed reduced expanded diameter, which was a key metric in the FBI’s calculations, and is the likely suspect in a roughly 30 percent reduction in cavity score.
Obviously, this has all been fixed in today’s bullet designs, but at the time, it was a real puzzler.
Pitfalls of High Magnification
When writing about the history of forensic firearms identification recently, I had planned to include an incident where using photographs alone had failed. However, I could not find it in my books. I asked an old colleague, examiner John Murdock, if he remembered it. He could not, so I left it out. A few weeks ago, John let me know he’d found it. It was in one of the books I had, but not in the history section.
The case was in Jerusalem well before comparison microscopes existed. One examiner had taken photos of the rifling marks on test bullets from a suspect revolver and a bullet from a crime scene. He enlarged them to an estimated 300X and proclaimed before the court that the bullets were undoubtedly from the same firearm.
A second examiner also had examined the same evidence. He asked the prosecutor to have the first expert count the land impressions on both sets of bullets. One set had three lands and grooves; the other set had four. That’s absolute elimination of the suspect firearm. The errant examiner had been so eager to show off his photo prowess that he skipped the most important steps in the process of identification. It’s another example of what not to do.