February 27, 2019
This month’s theme of handguns took me back several decades to my crime lab experiences and evaluation of firearms safety systems. Most people associate “forensic ballistics” only with matching bullets and cartridge cases to a specific firearm, but evaluations for proper mechanical condition proved important. Our standards of proof for matches were rigorous, and at least two of us had to agree it was a match before the test was reported “positive” for the record. Therefore, defense attorneys changed their argument to “the gun went off accidentally!”
As court-qualified forensic experts, we had to handle those defense questions professionally and truthfully on the witness stand. To that end, we maintained a reference collection of more than 1,000 firearms, largely handguns, so we could learn safety systems.
Many firearms have a “switch” that sets the gun to “Safe” or “Fire.” That’s a manual safety, one that the user consciously operates. Manual safeties usually ensure nothing bad happens if the trigger is inappropriately pulled. Revolvers have another class of safety: the self-acting safety that works without conscious thought on the user’s input and without rendering the firearm useless.
Most self-acting safeties protect against dropped-gun discharges. Here there are two scenarios that can cause an accident. The first is firing pin encroachment. If the hammer is at rest (not cocked), the force of falling can drive the hammer into the firing pin, discharging the cartridge. The second is jar-off. Here, the hammer or striker is in the cocked position and, if so equipped, the safety is set to “Fire” when the firearm hits the floor. Vibration jars the hammer and/or sear enough to cause the hammer to release.
Let’s focus on the encroachment issue first.
Before 1900 most revolvers allowed the firing pin to rest on a cartridge’s primer. That’s why so many old-timers knew to leave an empty chamber under the hammer. That workaround was useful for quality revolvers, but some of the “cheapies” did not have positive at-rest cylinder locking to keep the empty one under the hammer. They “free-wheeled,” potentially allowing a live cartridge to roll under the hammer.
Hammer notches were sometimes used, but they were unreliable against encroachment. Either the notch or the tip of the trigger could fail when 2 to 3 pounds of revolver hit the ground from waist-high.
In modern DA revolvers, preventing encroachment has two solutions. The first, a rebound system, positively moves the hammer away from the cartridge except when deliberate firing is about to happen. By 1900 both Colt and Smith & Wesson excelled in rebound systems for their DA revolvers, and as far as this old armorer is concerned, those systems remain state-of-the-art today.
Both use a machined piece of metal that’s aware of trigger position to automatically create a safe stand-off position for the hammer when the trigger is released. S&W uses a rebound slide that moves back and forth under a projection on the hammer. From 1898 to the discontinuance of the Python in 2003, Colt used a pivoting lever that accomplished the same thing. Both systems are robust enough to resist a significant force to the hammer, far more than a fall from waist-high can create.
Both firms later added a backup to the primary rebound system. In the remote possibility that the revolver falls from a huge height—think several stories worth of high-rise building—the force could theoretically be great enough to shear the sturdy hammer in two, overriding the rebound system. Should this near-impossibility happen, the secondary system remains in play to prevent discharges.
The other way to prevent encroachment is an interrupt system. Here, direct contact between the hammer and the firing pin is interrupted. Rather, an intervening part must complete the path from hammer to firing pin for firing to occur. This is a transfer bar system. The bar is linked to the trigger, and it only rises into position to transfer the hammer’s energy to the firing pin when the trigger is squeezed fully to the rear.
Many of us first learned about the transfer bar around 1973 when Ruger began using it in single-action models. However, it is much older. The Iver Johnson company fitted some inexpensive revolvers with a transfer bar system as early as 1894 with the tagline “Hammer the Hammer,” touting the virtual impossibility of a dropped-gun discharge.
Other encroachment-prevention systems in SA revolvers exist. Some fall into the manual category, including my favorite: leaving an empty chamber under the hammer.
Dealing with jar-off discharges in a cocked revolver is easy with modern DA designs. The rebound or transfer bar systems in double actions nicely handle jar-off episodes as long as nothing holds the trigger rearward and the internal lockwork has not been compromised. About the only thing that can interfere is a deliberately and grossly weakened trigger return spring.
For SA revolvers without a transfer bar system, the most common method to prevent jar-off is hammer intercept: catch the hammer before it discharges a cartridge. Old school works here: the hammer notches. As long as the trigger is free to move and the trigger return spring is reasonably lively, the sear end of the trigger will drag on the front of the hammer and find a notch, safely stopping the hammer. In the old Colt SAA, the sear has two opportunities to intercept the hammer: the loading notch and the “safety” notch.
The next time you pick up your favorite revolver, take a minute to reflect. Not only can we find a world of industrial history in the evolution of these unseen and unsung safety devices, but also we see a rich legacy of design and engineering excellence.