Choosing Self Defense Ammunition
It's a really common question. "What ammunition should I buy for self defense?" It's also a bit of a mire. Of course, it beats not thinking about it at all and just sticking the first range-blaster ammo you find in your gun and calling it "good."
Naturally, that still leaves questions. What ammunition should I use for self defense? What is the best self defense ammunition?
It is a very natural human notion to want to define "the best" and, in absence of personal expertise, to want an expert to tell one what is "the best." However, while the question may seem easy, the answer is not so straight forward. There are a lot of assumptions, sometimes conflicting, about what constitutes "right" and "best." There is also a lot of preamble to the subject.
[What ammo is right for my gun]
The first thing which must be learned is what ammunition your gun was designed to shoot. You probably already know this but let's go over how to find out any way.
The first place to look is on your gun. The ammunition specifics will usually be stamped somewhere quite visible, often either on the slide, on the barrel where exposed by the ejection port, or on the frame (particularly for revolvers).
Another place to look is in your Owner's Manual. The manual will typically state which calibers your gun was manufactured to use. Very often the model will have specific designations for different calibers. For instance Model ABC9 might indicate a 9mm chambering whereas Model ABC40 might indicate it is chambered for .40S&W. Always read your Owner's Manual prior to buying ammunition or attempting shooting the gun for the first time. If you do not have an Owner's Manual for your gun, check the manufacture's website. Often manufacturer's will have a PDF version available for download. Alternately you may consider writing or telephoning the manufacturer who may send you a physical copy for free or at a nominal price. For firearms which the manufacture no longer exists, frequently there are still PDF copies created and uploaded to the internet by a fan or special interest community.
To make sure that ammunition you purchase matches your gun's design, you can look on the box which the ammunition comes in. It should be printed clearly on the box and follow accepted naming conventions for that cartridge. You can also look at the base of the metallic cartridge itself. Usually there will be the same cartridge designation stamped on the bottom of the metallic case.
Some common cartridges used in the U.S. may have more than one name in usage. Here is an incomplete list of a few of the more common:
- 9mm, 9mm Luger, 9x19, 9mm NATO, (and certain combinations of these)
- .45ACP, .45 Auto, .45 Auto Colt Pistol (not to be confused with the incompatible, .45 Colt below)
- .45 Colt, .45 Long Colt
- .38 Special, .38SPL (not to be confused with .38 ACP or .38 Auto)
- .357 Magnum, .357 Mag (not to be confused with the .357 Sig below)
- .357 Sig, .357 SIG, (unofficially sometimes 9x22 Sig)
- .380ACP, .380 Auto, .380 Auto Colt Pistol (not to be confused with .38 ACP or .38 Auto)
- .22 Long Rifle, .22LR (not to be confused with .22 Long or .22 Short).
We should also briefly look at pressure designations. +P is an official SAAMI (Sporting Arms and Ammunition Manufacturers' Institute) designation indicating that a cartridge exceeds the maximum safe pressure of a "standard pressure" designation and typically develops somewhere around 10% more pressure. +P+ is not an official SAAMI designation and indicates that a cartridge develops pressure greater than even the official SAAMI +P designation. Only use +P rated ammunition in guns which the manufacturer has cleared to use +P ammunition. Using +P ammunition in guns not rated for it can cause accelerated wear, damage, possibly catastrophic failure, to the gun, and may void the manufacturer's warranty. Most experts advise using caution when deciding whether or not to use +P+ ammunition and certainly to avoid its use in guns not rated +P or for which the manufacturer specifically warns against use.
.22 Long Rifle typically comes in 4 pressure ratings: Subsonic, Standard Velocity, High Velocity, and Hyper Velocity. Most modern guns chambered for .22LR can safely shoot up to Hyper Velocity. However there are some guns which the manufacturer specifically advises using only Standard Velocity or lower.
There are some cartridges which SAAMI does not have official +P designations for. The .380ACP is an example of a cartridge which SAAMI does not have an official +P designation for. There are no ammunition standards body which has a +P designation for .380ACP.
Always verify with your manufacturer's documentation what ammunition your gun is designed to use.
[So what exactly is "self defense" ammo anyway?]
"Defense" ammunition is generally ammunition designed and marketed to incapacitate or kill an attacker more quickly and effectively. There are several different ways commonly used to try to achieve this effect. Often much of the technological effort is put into the bullet itself; which is the projectile portion of the cartridge, sent flying at high speed into the bad guy.
The first, and most common, method is expanding point bullets. The increased "stopping power" of expanding bullets was noted at least as early as the U.S. Civil War, if not earlier, where the new Minie Ball ammunition was noted to expand and deform dramatically as it passed through human bodies, creating horrific wounds. In the Hague Convention of 1899, the British Ambassador noted that their adversaries in India and Africa were frequently not stopped by their small caliber, non-expanding, bullets, which passed through them leaving little damage. He argued that the expanding bullets, which the British pioneered at their manufacturing plant in Dum Dum, India, were much more effective at "stopping" their opponents. Expanding point bullets typically come in two types, soft point and hollow point. Soft point bullets features a bullet in which the core material, usually lead, is "solid" but soft enough so that it will mushroom and expand upon impact similar to the way that play dough expands when smashed. Hollow point bullets feature a cavity, usually open, in the nose of the bullet. As tissue material fills and presses into the cavity, the pressure forces the cavity to spread apart like the petals of a flower opening. Some hollow point bullets feature a special ball or cap inserted into the cavity which acts as a wedge, forcing the cavity open. This is because some hollow point designs were noted to not expand if clogged with intervening barrier material such as fabric from clothing. The expanded bullet, because of its larger diameter, creates an equally larger wound channel, which does more tissue damage, hopefully leading to quicker incapacitation of the bad guy.
[Georgia Arms 9x19 Luger +P 124gr Expanding Point "Self Defense" ammunition]
Another popular method of increasing "stopping power" is to use frangible bullets. These are bullets designed to break into multiple fragments which create a greater number of smaller wound channels. This hopefully increases the possibility of one of those fragments damaging a vital organ or artery/vein and allows more channels to cause incapacitation due to unconsciousness because of loss of blood pressure through blood loss. There are three common types of frangible bullets used for defensive ammunition; compressed, segmented, and sintered. Compressed style bullets were pioneered by Glaser with their "Safety Slug" ammunition. Compressed bullets use a cup filled with small pellets, compressed into a near solid, and capped. The bullet is designed to break apart on impact, allowing the pellets to scatter in an expanding cone of tissue damage. Segmented bullets have been used for a very long time by both hunters and the military and date back at least to British research at the Dum Dum Arsenal, if not earlier. The concept is that the bullet starts as a solid projectile and is deeply scored or cut so that upon impact the bullet will break apart into pieces, following different wound channels. Sintered bullets are a somewhat newer technology, similar in some ways to compressed bullets. The sintering process takes a metallic powder and, under high heat and pressure, near welds it together into its final shape. The resulting product is durable enough to be loaded and fired, but is fragile enough to shatter back into dust upon impact, again creating an expanding cone of tissue damage. Frangible bullets are generally thought to be somewhat safer to bystanders because the "break apart" nature of the bullet greatly reduces dangers associated with ricochet and because the fragments, pellets, or powder, more rapidly lose their forward momentum in the tissue of the bad guy, thus reducing chances of over-penetration. It is thought by some that this also may contribute to enhanced "stopping power" by allowing the bullet do "dump all of its energy" into the bad guy, "not wasting any on over penetration."
[Glaser Blue frangible in gel]
[RBCD frangible in gel]
The reduced risk of ricochet from frangible bullets has spurred the development of sintered ammunition specifically intended for use on the target range. The purpose of this type of frangible ammunition is not defense.
There is a new and exciting field of research in self defense bullets. These bullets are not designed to expand. Instead they are designed using modern computer aided fluid dynamics modeling techniques. These bullets, often made of solid copper, have large cuts or scallops designed in the nose of the bullet. These scallops are designed to funnel and focus tissue fluid, harnessing it to increase tissue damage similar to the way that a spoon left in the bottom of the sink will inevitably redirect water from the facet above it, squirting a jet of water all over your shirt. Two new designs in this field harness the fluid dynamics slightly different. One has its scallops designed to balloon out in front of it, creating a much larger initial wound cavity than would otherwise be possible with traditional expanding point or solid bullets. The other design has its scallops designed to direct multiple "jets" of tissue and fluid at essentially right angles along the wound path as it travels through tissue. This has the effect of creating a much larger wound path or wounding channel than would otherwise be possible with traditional expanding point and solid bullets. The most excitement for this new technology seems to be directed at calibers which often lack the energy to both penetrate deeply and also expand, most notably the .380ACP.
Another, less common, method of increasing the wounding potential of a bullet without being expanding point is to design the bullet so that, though being accurate and stable in flight, there is an increased chance that the bullet will tumble once it enters media or tissue. Certain centrifugal spin rates coupled with bullets that are "longer" than they are "wide" tend to tumble easier. One noted example of this phenomenon was the 5.56 ammunition used in the Vietnam era M16's, though it is widely accepted that this effect was unintended and a mere coincidence. The effect of the "thin" or "narrow" bullet tumbling through tissue is, again, to increase the diameter of the wound channel by the length of the bullet instead of by its diameter. .32ACP ammunition, a common "pocket gun" caliber, is also noted to have an increased propensity to tumble once it enters tissue. A lot of military research has been invested in this effect and it is believed that several bullets from multiple nations were designed to enhance the chances they will tumble upon impact in an effort to increase wounding potential without running afoul of the Hague Convention, which prohibits signatory nations from using expanding point ammunition in warfare. Some non-expanding ammunition is deliberately altered by end users, hand-loaders, or, rarely, by manufactures to induce tumbling upon impact by manufacturing or grinding a flat or shallow concave cut offset into the tip of the bullet. This gives a visual impression that the bottom of a spoon was pressed into the tip of the bullet at a 90 degree angle. Once it enters tissue or media, the resulting pressure from that single scalloped cut causes the bullet to nose over and tumble, thereby increasing wound channel diameter.
["Spoon Tip" bullet]
It should be obvious at this point that much of the effort to increasing the lethality of a bullet, and therefore its ability to stop an attacker, focuses around increasing the size or number of wound channels. However, it should also be noted that most experts believe that the most effective ammunition must penetrate deeply enough to damage major internal organs, which causes greater internal bleeding and therefore quicker loss of consciousness. While bigger/more wound channels are better at stopping because they're more likely to damage one or more internal organs, a very large but very shallow wound is far less likely to cause a stop. One method of trying to achieve deeper penetration while still maintaining bullet expansion is to increase the velocity of the bullet. Most handgun bullets are comparatively limited in how much velocity can be achieved for a given cartridge. However, there are some ways of doing so. One of the easiest is to increase the pressure during the firing sequence. More pressure pushing the bullet forward, through the barrel, means more velocity imparted to it. This is why +P and +P+ "overpressure" ammunition exists; to increase velocity and therefore penetration depth.
Some some cartridges, often considered "smaller" or "weaker," are noted to be particularly difficult to develop a round which with both penetrate to the desired depth in tissue and still expand. For many of these, it is difficult to simply increase the pressure and still be safe to fire in most pistols designed to chamber it. In these cases, many experts recommend avoiding expanding point ammunition, which may fail to expand or, if it does expand, may fail to penetrate deeply enough. Instead they often recommend using standard solid, non-expanding, ammunition in order to achieve the desired penetration depth, believe it is better to sacrifice the size of the wound channel for a better chance of the smaller wound channel still intersecting a vital organ. Pistol cartridges with reputations for this include .380ACP, .32ACP, .25ACP, and .22 Long Rifle.
[Winchester 71gr. Full Metal Jacket .32ACP bullets]
[What is the FBI protocol and does it apply to selecting ammo for my gun?]
In the text above, I've mentioned several times the concept of wound channels and minimum penetration. But what is considered "minimum penetration" and where does it come from? Today, many Firearms For Self Defense experts recommend ammunition which will reliably penetrate at least 12 inches in calibrated ballistic gelatin. This is because that is the what the FBI concluded three decades ago in the now famous FBI "protocol." Well, sort of, anyway.
What is the FBI protocol? On April 11, 1986, bank robbers and universally reviled douchebags Michael Lee Platt and William Russell Matix, may their names be forever cursed, engaged in a gun battle with 8 FBI agents resulting in the death of two agents and leaving 5 other agents wounded. Douchebag murderers Platt and Matix sustained multiple grievous injuries but managed to fight on during which time, they were able to inflict death and injury upon the FBI agents. The key here is that they received multiple gunshot wounds but continued to fight. The most well known is a wound received by Platt, inflicted by a 9mm Winchester Silvertip (expanding point), which traversed the length of his forearm and entered his chest before stopping a mere inch away from his heart. It was widely held that a failure of caliber and ammunition to affect effective "stops" allowed douchebag murderer Platt to stay in the fight, continuing to kill and wound FBI agents. In the aftermath of the gun battle and tragic losses of FBI agents, a method and testing protocol was developed for identifying and quantifying more effective ammunition. Effective ammunition was deemed to be those which could penetrate a minimum of 12 inches in calibrated 10% ballistic gelatin after first passing through certain barriers such as heavy clothing, interior walls, car door sheet metal, or angled car windshields.
...how does this apply to my gun? The FBI concluded, based in no small part on the almost-but-not-quite fatal wound received by Platt, that effective ammunition needed to penetrate at least 12 inches in ballistic gel; ballistic gel being deemed as a close facsimile of human muscle tissue. Seems pretty easy, right? 12 inches penetration. The FBI also, importantly, concluded that the ammunition must first pass through what many now consider to be extreme obstacles which are more or less unrealistic for non-Law Enforcement civilian self defense needs. Many Firearms For Self Defense experts now conclude that "effective" self defense ammunition must penetrate to a minimum of 12 inches in ballistic gel, after first passing through the most likely "barrier," that being "clothing."
It should be noted that the FBI is not the only government organization which has a minimum penetration depth, nor do they all agree. In May 1998, INS/U.S. Border Patrol held an "ammunition symposium" at the Firearms Training Unit in Altoona PA, which included representatives from the U.S. Army, various ammunition companies, and independent testing labs. Following two days of discussions a report was issued which included a 9 inch minimum penetration standard.
[What about studies?]
It is quite common for interested observers to wonder if there are any studies tracking "real world" performance of certain ammunition. The answer is "yes, but..." The "but" in this case being slightly different for each of studies, and there are "buts" for each of them. That said, a brief look at several of them and a broad overview of their conclusions may be beneficial.
Marshall and Sanow:
After the FBI tests, the most famous method of trying to quantify effectiveness of cartridges is probably the statistics compiled by Evan Marshall and Ed Sanow, published in multiple books. Handgun Stopping Power: The Definitive Study by Evan P. Marshall & Edwin J. Sanow, using the "empirical evidence" method, compiled the results of hundreds of real world shootings. An important part of their research was that they were looking at "stops" and not "kills" with particular emphasis on "one shot stops." That is, what is the percentage of times caliber was used and the bad guy stops attacking after one shot. What they found was that every caliber had effective "stops" after one shot, even the lowly .22 Long Rifle, which ranks at about 21%. Other calibers, ranked by brand and offering, rated 90% or greater, including certain offerings in .357 Magnum and .45ACP. The two big complaints about Marshall and Sanow's statistics are first, that a "stop" just means, "stops attacking" without regard to why, which might include, "I don't want to get shot by ANYTHING" instead of being forced to stop by damage, which would not be effective on worst case attackers such as those so enraged as to not feel pain or those under the influence of narcotics. And, second, that neither the ammunition nor the shooting incidents can be statistically controlled and the events are individually unique. Further, the issue of limited sample size is thought by many to invalidate the results.
In 1904, the U.S. Army conducted a series of tests to determine the most effective military handgun caliber. Several different standard military calibers were tested, including 9x19 Luger, .45ACP, and .38SPL. In order to define the effectiveness of the ammunition, live cattle and human cadavers were shot. Cadavers were hung and shot. The distance that the shot made the cadaver swing was considered indicative of the effectiveness of the shot. The cattle were shot through the lungs, in two tests, the first test being one shot, and the second being multiple shots, and the time required for the animal to die recorded. The Army eventually concluded that a minimum of a .45 caliber bullet was the most effective in affecting kills. This test, though widely reported by fans of the .45ACP, is also generally considered to have many important scientific problems. These problems stem from two directions. First, humans aren't cattle and measuring time to death of a cow may have little to no impact of that of a human. Second, how far a human corpse swings under "impact" of a bullet is now thought to have little to no impact on its ability to kill. Further, as with the Marshall and Sanow results, it is thought that extremely limited sample size invalidates the results.
The Medium is the Message: Firearm Caliber as a Determinant of Death from Assault:
In 1972, Franklin E. Zimring of Berkeley Law, published an article in the Journal of Legal Studies. This study was a study of deaths by caliber in Chicago in 1970. The intention of the study was to attempt to quantify lethality by caliber with the thesis that larger calibers are more lethal and to quantify homicide by motive (intentional or unintentional). The study tracked shooting in five categories of calibers, .22, .25, .32, .38, and greater than .38. The study concluded that the larger the caliber, the more lethal it was, thus 45's were more lethal than .38s, .38s more lethal than .32's and so on. However, an important note is that, when combined, .22 & .25 account for around half of all fatalities, outnumbering by far any other individual caliber in the set. This is not because .22 and .25 are more fatal in this study, statistically, but rather that a much greater number of people were shot with .22 and .25. A greater percentage of those shot survived than that of "larger" calibers. However, the raw numbers of those killed with .22 and .25 were greater. This is most likely where the commonly repeated wisdom that "more people have been killed with the .22 than any other caliber" comes from. The most important issues with this study for us today surround its age and its location. First, the study dates from 1970, nearly half a century ago, and therefore does not include any of the improved bullet technologies which have advanced since 1970. Second, the study is very limited in its source, covering only Chicago and therefore has issues when expanding the findings to the rest of the U.S., never mind when expanding those findings to the whole world.
The Pig Board:
In 1928, the U.S. Army convened what has become known as The Pig Board in order to determine the most effective rifle caliber. Tests were conducted by shooting anesthetized pigs at distances ranging up to 1,200 yards with special attention on results at 300-400 yards. The Army found that the ideal caliber was between about .27 and .3 inches, recommending John Pederson's .276 Pedersen cartridge. Most obviously, this study was conducted on rifle cartridges generating muzzle velocities and muzzle energies which could only be dreamed of by most handgun cartridges. However, it does reinforce the common opinion that greater energy levels delivered to the target are more likely to affect stops and kills.
The Strasbourg Tests:
In 1991, a report was widely published, reported to very scientifically quantify the incapacitation capability of various handgun ammunition. The test subjects were a breed of goat known as the French Alpine Goat, an animal both common in the U.S. and popular with the military at the time for lethality tests. The test concluded that frangible ammunition such as Glaser Safety Slugs, provided the most reliable and effective results. While widely lauded at the time of its release, several noted experts were quick to cast aspersions on the report, labeling it as a hoax or a fraud. There are still questions as to the origin of the report and, if the tests actually occurred, who it was that funded the tests. While it has been difficult to determine for sure, many today accept that the test probably did happen and likely did report the results honestly. Further many believe that the tests are similar to what the U.S. Military would have expected and they suspect that the U.S. Military was, in fact, the origin of funding for the test.
An Alternate Look at Handgun Stopping Power:
On July 8, 2011, officer Greg Ellifritz posted his own study on the webside of Ohio advocacy group Buckeye Firearms Association. This study, titled An Alternate Look at Handgun Stopping Power, too, a methodology similar to that of Marshall and Sanow in that it looked at "real world" shootings. Ellifritz compiled data from first hand accounts, police reports, autopsies, and other sources. Ellifritz's compilation is different from that of Marshall and Sanow in several important ways. While Marshall and Sanow only included data from a single shot to the torso, Ellifritz included multiple shots and non-torso hits, including the head and extremities. He compiled data into results categories which include: number of people shot, number of hits, percent of hits that were fatal, average number of rounds until incapacitation, percent of people who were not incapacitated, one-shot-stop percentage, accuracy (head and torso hits), and percent actually incapacitated by one shot (torso or head hit). These expanded categories give a more sophisticated view of the information and help remove the "stops" effected by "I don't want to get shot by ANYTHING" but still suffers from from issues related to how his data was sourced. Like Marshall and Sanow, Ellifritz also found unexpectedly high percentages of "stops" from calibers generally considered "weak."
So what do all of these conflicting studies mean? Well, it's hard to say for sure. The most important take-home is that generating information on the "most effective" cartridge and caliber is really hard and even experts with a research background and a vested interest in finding "the best" have a difficult time doing so. There are some things which almost all studies agree on, however.
First, nearly any caliber and cartridge can affect stops. From the lowly .22 Long Rifle up to the revered .44 Magnum, everything can, and has, killed bad guys and made them decide they had better places to be. Psychosomatic effects ranging from the "I don't want to be shot by ANYTHING" effect through the "TV has programmed me to believe I must drop to the ground and flop like a fish if shot (at)" effects are very real, very effective, but not necessarily always reliable across the board.
Second, most studies seem to concur that larger caliber bullets are sometimes, but not always, more "effective" than smaller caliber bullets. It's hardly a "hard and fast rule," but more of a generality and may not always take into account advances in modern, expanding point, ammunition.
Third, most studies seem to agree that higher Muzzle Energies generally equate to greater tissue damage to the bad guy.
Fourth, most studies seem to agree that cartridges which create larger wound paths and wound cavities tend to be more consistent at stopping and killing. This seems to be because a larger wound channel is both more likely to cross an important internal organ or an important blood vessel or artery, and that a larger wound channel is going to bleed more. More bleeding, internal or external, will equate to quicker "stops" because the drop in blood pressure leads to unconsciousness.
It is also generally held that deeper penetration coupled with greater Muzzle Energy and larger wound channels increase the, already slim, chances of disrupting or destroying the central nervous system (usually the brain or the spine) which will cause immediate "stops."
[I've been told to just use whatever my local police use]
Many people are told to find out what their local Police Department issues and to use that caliber and specific cartridge. On the face of it, this seems to be a reasonable enough proposition. After all, who would know more about what ammunition stops a bad guy than the people charged with stopping bad guys? However, there may be unseen forces affecting what ammunition and caliber the Police Department chooses. Most of the time the decision of the gun, caliber, and ammunition are influenced, perhaps even decided by, what other police in the nation are using. "Are the cops in New York using it? Well then so are we!" The decision is also often affected by discounts from manufacturers to the Police, the so called "Blue Discount." Glock, a brand of widely respected handguns, had difficulty gaining acceptance for their newfangled "plastic" framed handguns but offered deep discounts to police, who bought them. The "just use whatever the cops use" guaranteed that the firearms public would soon buy Glocks as well. There may also be budgetary constraints, particularly in today's reality of shrinking budgets, which could force police purchasing agents to buy "less effective" ammunition merely because it is all that they can afford.
It has also been suggested that, in the event that you ever are forced to shoot a bad guy in self defense, an over-eager prosecutor may try to argue that you used unusually deadly bullets in your gun, if you use expanding point ammunition, and therefore your mind set wasn't that of self defense but, instead, you were actively hoping and looking to kill someone. The remedy suggested is to just use the same caliber and ammunition as police. However, the instances of a prosecutor trying to use the "unusually deadly" argument in regards to commercial ammunition is quite rare. The actual defense against this is not to just use whatever the local cops use, but, instead, to exercise sound judgment about using a firearm in self defense. While you should consult your local laws because they can vary, the essence of it is to only deploy your firearm when you, or someone you are certain is an innocent third party, are in immediate and clearly articulable danger of death or serious bodily injury. If there is no danger, such as if the bad guy is fleeing or if you just want to try to "warn" him, then don't deploy the firearm and don't pull the trigger. Justifiable Deadly Force is the defense, not "the cops use the same ammo."
[I've heard that I shouldn't use reloads because a prosecutor might call them "super-deadly"]
It is often repeated in conversations with people who hand load custom ammunition that they should only use their hand loaded product for purposes other than carry and self defense. The reasoning is that, similar to the above described over-eager prosecutor, some prosecutor looking to "stick it to" you will argue that use created unusually deadly ammunition and therefore your mind set wasn't that of self defense but, instead, you were actively hoping and looking to kill someone. They often cite the case of Dan and Lise Bias. Mr. Bias hand loaded some very light .38SPL ammunition for his recoil sensitive wife who eventually committed suicide with them. The prosecutor claimed murder and used gunshot residue tests from commercial ammunition to "prove" that the deadly shot must have been fired at a fair distance away, instead of point blank as the defense claimed, and therefore was murder. Mr. Bias was eventually acquitted but only after a long and financially ruinous trial for a grieving widower. However, this case had nothing to do with self defense. It was a suicide. Further, cases where hand loaded ammunition have been an element are exceptionally rare. Again, the actual defense against this is not to avoid using hand loaded ammunition for self defense, but, instead, to exercise sound judgment about using a firearm in self defense. Again, only deploy your firearm when you, or someone you are certain is an innocent third party, are in immediate and clearly articulable danger of death or serious bodily injury. If there is no danger then don't deploy the firearm and don't pull the trigger. Justifiable Deadly Force is the defense, not "I used commercial ammo."
My personal conclusions are not always popular with many gun experts, both actual experts and self-proclaimed. Based on the surprisingly high percentage of even "mouse calibers" affecting actual "stops," I conclude two requirements.
First, your gun and ammunition must be reliable. Your gun must reliably function and your ammunition of choice must reliably function in it. Your gun and ammunition must go "bang" every time you deliberately pull the trigger. Your gun and ammunition should only go "bang" when you pull the trigger, not when dropped, or because of defect of design, manufacturing, or wear.
Second, you must be reliable. You should only deploy your gun when there is a genuine and articulable threat of death or serious bodily harm to yourself or an innocent third party. Further, only the bad guy and not innocent third parties should be shot. Only shoot when you're justified and only shoot what you are justified in shooting.
You must use your best judgment, sifting through sometimes conflicting research and reports, to make an informed decision on what will be most appropriate for you. It is important for you to remember that most people are non-Law Enforcement civilians (not military). Our goals are to make the bad guy go away and stop trying to hurt us, not to arrest or to kill; stop.