Rule 3: Keep your finger off the trigger until you are ready to shoot.

Mechanical failure as mentioned would have occurred by this point, and the only remaining action that can cause the gun to fire is a pull of the trigger. Following Rule 3 is taking the final step in the prevention of a negligent discharge – if the operator has reached this point, Rule 3 is the last line of defense. Failure to keep his finger out of the trigger guard means that there is nothing left to prevent him from unintentionally firing the gun. The number of ways in which this can occur are legion.

Are there circumstances in which Rule 3 is violated for reasons other than negligence? I can think of two possibilities. The first is dry-fire practice, which is a common and accepted method of training such essential skills as hold, trigger pull, stance, sighting and breathing. I would argue, however, that dry-fire practice, when properly performed, is not a violation of the Four Rules, because Rules 1, 2 and 4 are being followed (the gun is pointed at a suitable backstop) and the shooter is indeed ready to shoot. The only difference is, he has removed all live ammunition from the area and verified the condition of the chamber multiple times. In all other respects, his actions mirror those of taking a normal shot, and therefore for safety purposes it may be treated as such.

The other case that comes to mind is disassembly. Many bolt-action guns, for example, require that the trigger be pulled to remove the bolt. This is of course performed with the bolt in the open position, rendering a ND physically impossible, but this is still no excuse for laxity. If a person is intentionally violating Rule 3 in this manner, his failsafe is Rule 2. The same is the case when disassembling a Glock, which requires that the trigger be pulled with the slide in the closed position in order to take apart the pistol. As has been unfortunately and graphically demonstrated on several occasions, this design can be particularly unforgiving if Rule 2 has also been violated. These are the only cases of necessary tasks I can recall that require a violation of Rule 3. Others may exist, but they are surely exceedingly few in number.

Rule 4: Be sure of your target and what is beyond it. This interpretation given in the UF Ordnance Corner header is completely correct in the context of a deliberate shot. You never shoot at anything until you have positively identified it. You never fire at a shadow, or a sound, or a suspected presence. You shoot only when you know absolutely what you are shooting at and what is beyond it.

In the context of a ND, however Rule 4 assumes that it has already occurred and makes a final attempt to contain its consequences. The shooter has violated a sufficient number of Rules to result in the ND, and he must now deal with the aftermath of his mistakes. A violation of Rule 4 may multiply those consequences a hundredfold. There are no circumstances requiring the routine or deliberate violation of Rule 4.

Firing a gun in self-defence may require a violation of the second half Rule 4, but this is neither routine nor deliberate. It is a highly specialized and extreme exception; it is an emergency. The responsible gun owner who prepares himself for such an event recognizes this and does everything he can to mitigate the fact that he may be forced to fire at a target against an unknown background, hence the concern with such important things as overpenetration in defensive cartridges.

As we have seen, the Four Rules function as a system of redundant failsafes. They work first to prevent a ND from occurring, and in the event of one, to minimize the damage it causes. The number of violations required for a ND to cause damage is always greater than the number of violations required to simply produce a ND. The number of instances in which normal, routine and safe gun handling requires a Rule violation decreases drastically as we move down the list, ending in zero.

The Four Rules are a system of safeties within the larger framework of safe firearm practices. They are not a comprehensive list of everything a person dealing with firearms should and should not do. They are worded shortly and simply for ease of understanding and memorization. They are not Alpha and Omega, they are a groundwork for a fuller understanding of safety, for as we have seen, there are common circumstances where violation is not only acceptable, but necessary.

As seen above, the Four Rules pertain to firearm handling and to handling only, and for this reason items such as DesertRat66's rule 5, “Be sure your guns are never accessible to unauthorized or untrained individuals,” and rule 6, “Alcohol and Guns in any quantity DO NOT mix,”although important, were not included in the original Four Rules. Both of these deal with attributes of the person handling the firearm: his condition, level of training, and so on, and not with the actual handling of the firearm itself. Likewise, they also play no role within the system of failsafes that is the Four Rules. Being authorized, trained or sober are all criteria that should be met, and while they may reduce the likelihood of a ND, they will not prevent or contain one in any way not already accomplished by adherence to the Four Rules.

The same applies to any number of other important safety procedures. Using the correct ammunition for a particular gun is vitally important, but has almost no bearing on NDs - injury or damage resulting from this is normally the product of the gun being fired exactly when intended. Eye and ear protection, while also vital, do not concern the actual handling of the gun.

The Four Rules are, to a great degree, a complete and enclosed system of protections. This does not mean the individual Rules are flawless: as we have seen, non-negligent violations are often required, but the entire system is self-supporting and accounts for violations. When they are discussed, suggestions for improvements or additions are often brought forward. While well-intentioned and often referring to other important gun safety practices, the suggestions I have seen do not contribute to the functioning of the system of the Four Rules, and to simply hang them upon the end weakens an important aspect of the Four Rules: brevity, and by extension memorability. For these reasons, such proposed additions are and ought to remain, so to speak, non-cannonical.