There are two issues related to the small percentage of law enforcement agencies that moved to adopting DAO revolvers and later DAO pistols.
First was the bad habit of officers pointing a revolver at a suspect to cock the hammer leaving the revolver in SA mode. Under extreme stress and in a situation where the officer might be startled it’s far easier to unintentionally shoot the suspect with an SA trigger than a DA trigger.
While firearms forensics cannot determine whether a revolver was fired SA or DA, eye witnesses at the scene (often other officers) and surveillance video certainly could.
The idea behind the DA trigger on a revolver (the vast majority of which lack any other safety device other than a transfer bar) is that the long and heavy DA trigger helps prevent a negligent discharge.
In addition, when it comes to reholstering a revolver (or for that matter a DA or SA/DA pistol) putting your thumb behind the hammer gives you an obvious tactile warning that the hammer is being cocked if something is intruding into the trigger guard (including a nervous officer’s trigger finger after a high stress situation).
One method of preventing officers from cocking the revolver (or a SA/DA pistol) was to special order the revolver or pistol as DA only.
Second, a small percentage of misguided officers, clearly instructed not to cock the hammer and to always fire DA (whether they carried an SA/DA and DAO revolver or pistol) had a bad habit of staging the DA trigger, by pulling the DA trigger just short of sear release and then holding it there. That was far, far worse than cocking the hammer, as it can be hard to actually stop just short of firing, it varies by make and model of revolver. For example the more mechanical feel of the trigger on the Ruger Security/Service/Speed Six series made it easier to do than with the average S&W revolver but in all cases it could vary from chamber to chamber. Staging a DA trigger with 100% reliability is hard to do even in ideal conditions where you are not under stress or time pressure. It’s virtually impossible to do it reliably under stress.
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Striker fired pistols like the Glock quickly supplanted and then replaced the SA/DA and DAO revolvers used by many law enforcement agencies. In many cases they were seen as ideal as the belief often was they required less training. There was no safety to mess with, and there was also just a single trigger pull to master. At a basic level it was just draw and pull the trigger, much like a DA revolver.
The problem was that the striker fired pistols had lighter and shorter trigger pulls. They were not as short and light as an SA pistol like the 1911 or Hi Power, but were much shorter and lighter than a DA revolver. More accidental discharges occurred and the Glock was over represented due to both its numbers relative to other striker fired pistols and the requirement to pull the trigger to disassemble it. Improperly clear it and it will go bang.
Some departments ordered “New York” triggers in their Glocks with a heavy (around 12 pound) trigger pull to provide some of the ND resistance of a DA revolver, although the trigger pull is still shorter.
If you look at police duty holsters back in the 1970s and 1980s and compare the, to modern duty holsters you’ll notice many duty holsters today ride out away from the duty belt while back in the day they were in close to the hip.
That’s largely an artifact of striker fired pistols and the greater potential for a negligent discharge if anything intrudes into the trigger guard while being reholstered.
It takes a lot less force and a shorter push on the striker fired holster to get an ND than with a longer heavier DA trigger, and none of the safeties on the striker fired pistol do anything to prevent it when they are all disabled by pressure on the front of the trigger. Putting the holster farther out reduces the potential for clothing related obstructions and it helps ensure the bullet will miss the officer’s leg, or at least a major artery, if they have an ND.
Several years ago a device was developed for most Glock pistols (not the 42) where the shooter can press on the back of tue slide to prevent the striker from fully cocking if the trigger is pulled while it is being reholstered. It’s much the same as putting your thumb behind the hammer on a DA revolver or pistol, except it doesn’t so much provide a tactile warning as it does prevent the pistol from fully cocking and firing.
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We might as well cover SA pistols like the 1911 and Hi Power as they are also in wide use. With the 1911, you can still hold the pistol pointed at the suspect safely. You just leave the manual safety on, with the thumb on top of the safety. You train to place the thumb on the safety as you draw and then depress it when you are ready to fire. Whether it’s a 1911, a Hi Power or a CZ75B carried cocked and locked, order isn’t important as if the trigger is pulled before the safety is depressed, the gun will fire when the safety is depressed.
When reholstering you train to place it on safe, but then back stop that by putting your thumb over the back of the hammer, but not to depress it. That not only controls the hammer but with an 1911 also lifts the web of your hand of the grip safety which also prevents an ND.
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For an armed citizen with a DA revolver or DA pistol with the hammer cocked the biggest issue is the potential to shoot when you don’t actually need to shoot, such as if you draw and cock the gun and the assailant suddenly surrenders or retreats. You are only 3-4 pounds away from an unjustified use of lethal force.
There’s also a lesser issue of having time to cock the hammer back in the first place. If you do that while the assailant is trying to kick down your door and you are telling him “stop or I’ll, while you are calling 911 it’s not an issue, unless of course you then accidentally shoot while the assailant is still outside. And remember, the unintended discharge was the big issue with officer involved shoots with cocked hammers. Unlike an officer, you do not enjoy any kind of limited immunity, sovereign immunity, department attorneys, or the latitude the court has historically extended to mistake of fact shootings and by stander shootings.
If however, you have time because the assailant is still 50 yards away from you and you shoot, it’s going to be hard to demonstrate the assailant was an imminent threat regardless of whether you fired DA or SA, but having the hammer cocked just makes it look worse.
If you cock the hammer for greater accuracy as the assailant is 50 yards away from you and retreating, it’s a bad shoot and cocking the hammer just makes it more egregious.