These are the rifle cartridges in common use by modern western militaries. The smallest is the standard NATO infantry round, 5.56x45mm. Adjacent on the left is the “medium” 7.62x51mm, also a common infantry round, especially in theaters where longer engagement distances render the 5.56 ineffective. Middle is .300 Winchester Magnum (.300WM), which has long been fielded for snipers needing to push beyond the 1000-yard “effective” range of the 7.62mm NATO. The .300WM is being supplanted by the fourth cartridge, .338 Lapua Magnum (.338LM), which has emerged as the top long-range military sniping cartridge. Previously, long-range snipers often relied on the largest of the “small arms” cartridges: the century-old “heavy” .50 Browning Machine Gun (.50BMG) round.

The following table lists the size, weight, and range of each cartridge for typical military loads, barrels, and sea-level atmospheric pressure. The point at which bullets slow through roughly 1100fps is a common benchmark for range because that is the speed of sound at typical air temperatures. Historically the accurate range of a precise bullet has been limited by the effects of crossing through the sound barrier. However, modern barrels tend towards faster rifling twist rates which increase transonic stability. In the last decade snipers have recorded first-shot kills at ranges where their bullets were subsonic. Snipers at high altitudes have made a number of remarkable kills at distances of up to 2700yds. The thinner air at high altitudes creates less drag on bullets and thus extends their range.

On the heavy end it’s interesting to see that the .50BMG is actually at a disadvantage to .338LM in terms of range (not to mention the added weight of the rounds and heavier guns needed to efficiently shoot it). But it does have the capability of delivering more than double the payload, so it is still in use for anti-materiel roles.

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If you’ve read some of the recent articles on ballistics, especially the comments on this one, you might logically deduce that for any gun the best bullet is the lightest you can find. After all, lighter bullets produce less recoil and more muzzle energy. How can you lose?

It turns out a number of companies have come and gone trying to exploit this argument to sell extremely light, fast bullets, especially for handguns. Liberty Ammunition is the latest on the scene. A detailed critique of previous ventures that includes extensive explanation of the problem with ultralight bullets is archived here.

The short answer is that very light bullets are very bad for defensive use because they lack penetration, and companies that sell them for that purpose are guilty of misleading advertising. Yes, at short ranges they “dump” more kinetic energy into targets, but that energy does not create the deep wound channels experts know are necessary to physiologically stop aggressive animals (including people) in typical shooting scenarios. That’s the end of the argument as far as consumers are concerned.

I will make two other esoteric observations: First is that on the low end ballistic efficiency decreases with bullet weight. The second is that sectional density decreases with weight, which means that ultralight bullets lose speed (and energy) faster and are more susceptible to deflection in flight.

So ultralights suffer impairments at every stage of ballistic consideration: internal, external, and terminal. At short range their extraordinarily high velocity does enhance penetration through some materials, but that does not mitigate their drawbacks. If you have a specific scenario that requires penetration you should get a rifle and load suited to it. Never load your defensive handgun with ultralight bullets!

Most firearm action components are made of steel, but they can be given a variety of coatings with remarkable properties. Shown here are four AR-15 bolt-carrier groups (BCGs). From top to bottom they are coated in:

• Phosphate, a.k.a., parkerized
• Nickel-boron, a.k.a. NiB
• Chrome
• Nickel-Teflon, a.k.a. NP3

Phosphate

Phosphated steel has been used in guns for over a century, and is still the military standard for BCGs. The one shown here is from a premium Noveske rifle. I asked Noveske why they don’t use a more modern coating and was told, “Even though the smooth hard coatings take a few seconds less to clean they do not hold oil. Oil keeps the gun running longer and keeps the gun from wearing out. The standard parkerizing holds on to oil longer than any other finish. It keeps the gun running longer.” Indeed, the finish on this BCG is rough and soaks up lubricant like a sponge, though it also seems like a magnet for grit and fouling.

Chrome

Chrome is widely used in barrels and chambers because it is extremely hard, corrosion resistant, and smooth. I’ve run the one shown here from a Ruger SR-556 more than any other single AR-15. It is easier to clean and requires less lubricant than a standard BCG. I suspect it should also be less prone to failures if insufficiently lubricated.

Nickel-Teflon

NP3 is a relatively soft coating of Teflon-impregnated nickel done by ROBAR. Due to its softness it will show wear, but due to its lubricity I am not aware of any instances of it wearing through. It is so slippery that any fouling can be wiped off with a dry rag. The BCG here is from a Barrett Rec7. Wilson Custom also uses NP3 on all their BCGs.

Nickel-boron

The industry leader in NiB coatings for firearms is WMD Guns, which markets the coating as NiB-X, and which spun off from FAILZERO, which markets it as EXO. Shown here is a new $200 mil-spec NiB-X BCG.

Both NP3 and NiB are lubricious enough to run without lubricants. (In fact, like dirt, lubricants can’t really adhere to these electroless nickel-based coatings.) ROBAR claims that NP3 is more consistently lubricious than NiB and significantly more corrosion resistant. NiB is hard enough that you can use steel scrapers and brushes to clean it. However it is not as smooth as NP3 or chrome, so you need a lot more than a cloth to remove the discoloration of carbon fouling that embeds itself in NiB’s microscopic surface irregularities. Granted, that’s a cosmetic issue, not a functional one. It does, however, lend plausibility to a few reports of NiB BCGs jamming up and needing lubricant to unlock.
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The .45 ACP cartridge has a strange, cult-like devotion among many American handgunners. Strange because it is practically inferior to more modern pistol cartridges. Its diameter makes it impractical for double-stack magazines (which is not to say those don’t exist: I handled FN’s double-stack FNP-45 but it is far too large for a carry gun), which means capacity is typically just 8 rounds for a full-sized gun. The chamber pressure limit for the cartridge is just 21kpsi, vs. 35-40kpsi for more modern pistol cartridges. This may have been a benefit in the past but with the ubiquity of strong alloys and precise manufacturing technologies it is no longer. It does, however, mean that the cartridge is optimized for propelling heavier bullets at slower velocities, which as I explained in a previous post results in more recoil for a given level of energy. Finally, the heavy bullets and larger dimensions of the cartridge make it significantly more expensive than other defensive pistol cartridges.

The .45 ACP was developed and launched along with the 1911 pistol, so even though 1911s are produced for other calibers there seems to be some nostalgia for shooting 1911s chambered in .45 ACP. But there is also a great deal of lore surrounding the prowess of the cartridge as a “man-stopper.” This is almost certainly rooted in the military’s experience with pistols. The U.S. has followed the 1899 Hague Convention, which prohibits the use of hollow-point bullets in warfare. Until 2010 even military police carried full metal jacket pistol loads. Rifle bullets go fast enough that standard copper jackets break up in soft tissue, creating large wound channels and dumping most of their energy into human targets. But slow, solid handgun bullets punch through human targets practically intact, which means that they leave a wound channel only as wide as the bullet itself. Given this constraint it’s not surprising that .45s developed a reputation for stopping people with fewer shots than smaller bullets that have been government issued.

But this situation does not apply to civilians, who can carry hollow-point bullets. The standard test for terminal ballistic performance is to shoot gelatin that is calibrated to the consistency of animal tissue. The following image shows the wound channel and penetration of standard loads using jacketed hollow-point (JHP) bullets:

With hollow-point bullets the .45 ACP offers no advantage in stopping power over .40 S&W or .357 SIG (my favorite), which have the advantages of lower recoil, lower cost, and higher magazine capacities.

One obvious question is why no .45 cartridge has been specified with higher pressure limits? For example, raising peak pressure to 32kpsi could add 200fps to the load shown here, boosting both Relative Energy and Relative Recoil to 1.7x. One obstacle may be the popularity of 1911 .45s: Neither a standard 1911 nor a standard .45 ACP case can safely support much higher pressures. A few wildcat cartridges have emerged over the years (e.g., .45 Super and .450 SMC) with the same external dimensions and higher pressures, but SAAMI will probably not approve any such cartridges because they could be inadvertently fired in a .45 ACP gun, causing damage and potentially catastrophic failure. SAAMI did approve the .45 Win Mag because its slightly longer cases wouldn’t chamber in a .45 ACP. However that caliber never really caught on, perhaps due to the other problem: power factors above 250 represent a level of recoil that is apparently not practical for a defensive carry pistol.

I recommend this page for further reading on terminal ballistics.

Yesterday’s post highlighted one gun cartridge (the .357 SIG) that, in small pistols, delivers energy disproportionate to its recoil. Today I will describe more generally the physics and practical considerations that go into optimizing a gun for a particular purpose.

The purpose of a gun is generally to project some combination of Energy, Range, and Accuracy.

Today this is done with firearms, which are subject to practical constraints on Length, Cartridge Size, Chamber Pressure, Rifling, and Recoil. Cartridge Size is a function of propellant (gun powder) capacity and projectile (bullet) size. To understand the physics that relate all these variables we will actually start with a Cartridge and work backwards, because:

A. Propellant volume puts an upper limit on the Kinetic Energy a gun can generate.

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Those who study the art of precision shooting know that today a good marksman will easily spend 5 figures on equipment, thousands of hours of practice and development, and at least a dollar per shot. All this with the singular objective of being able to project a few kilojoules of energy up to a mile or so away as quickly and accurately as possible.

Today the tool of the trade is the rifle: essentially a tube that converts chemical energy from a gunpowder-fueled cartridge into the kinetic energy of a spin-stabilized bullet. Riflemen go to great lengths to maximize how precisely they can get this machine to project a bullet into the atmosphere. But under field conditions accuracy is limited to about half a minute of arc (i.e., half an inch of error per 100 yards distance from a target). The physics of atmospheric ballistics require heavier equipment to reach further distances, and put the outer limit of a man-portable rifle’s range at about 1.5 miles.

Thanks to evolutionary advances in optics, ballistics, firearms, and cartridges, the capabilities of riflemen have never been greater. But the whole art seems somewhat archaic.

In time precision rifles will go the way of blackpowder muskets, replaced, I suspect, by optical firearms. This may at first sound like trite science fiction — am I seriously writing about the advent of rayguns? Well yes, but consider how and why this wouldn’t be as revolutionary as it sounds: The first optical firearms will probably fire chemical lasers.

Current single-shot chemical lasers are roughly 30% efficient at converting chemical energy into optical energy. They require supersonic mixing of reagents and generate a lot of heat. Current rifles are also about 30% efficient at converting the chemical energy of smokeless powder into supersonic kinetic energy, and also generate a lot of heat. From this point on optical firearms take the lead: A ballistic projectile begins to lose energy and accuracy due to atmospheric interactions from the moment it leaves the barrel. Optical energy follows a line of sight at the speed of light and loses energy only to the degree that its wavelength is dispersed by the atmosphere through which it travels.

Chemical laser firearms could be similar to current rifles in many ways. They will probably employ a single cartridge per shot containing not only the chemical reagents but also an explosive “primer” to mix them when fired. Instead of projecting a bullet into a rifled barrel for acceleration, the cartridge will project the mixture into an optical cavity where mirrors and lenses will focus the flash of high-powered light out the muzzle. Depending on the particular technology the reagents may leave the gun like a muzzle blast, or stay in the cartridge for ejection. Optical cartridges could be reloadable by shooters, just like ballistic cartridges.

So where’s the sport in that? If you can see it you can put a hole in it? As now, a lot of the critical cartridge development will be done by professional chemists at propellant manufacturers. Gun makers will be mostly replaced with the manufacturers who are already making the rugged, precision glass for 4-figure riflescopes. Perhaps marksmen will switch their focus to tricks for pushing the diffraction limit with man-portable optics to further range and accuracy?

.22LR is the smallest firearm cartridge in common use today. Common wisdom holds that it is too weak to use for defense against humans or for hunting any animal larger than a raccoon. However there is some evidence that this cartridge is underestimated.

.22LR ammunition is attractive for several reasons:

• It is very cheap and widely available. Quality loads can be found for under $.05/round.
• It can be very accurate. Match-grade loads from a suitable rifle can produce 1-inch groups at 100 yards. In fact it is used for many target sports, including a majority of olympic shooting events.
• It has negligible recoil and minimal muzzle blast, making it ideal for training youth or new shooters.

As I have noted elsewhere, it is a great round for shooting with suppressors: Because .22LR produces so little propellant pressure and volume, suppressors for the caliber can be made very small and light. Furthermore, there is an abundance of subsonic loads on the market, which allow for nearly silent shooting: When shooting slower loads out of my rifles with an Outback suppressor the only audible sound from the gun is the click of the sear releasing the hammer and striking the cartridge rim. The sound of the lead projectile striking a soft target even a hundred yards away is louder to the shooter. (Note that in moderate weather muzzle velocity has to fall below 1000fps to avoid sonic echoes, which increase in loudness and turn into unmistakable sonic cracks as muzzle velocities cross the speed of sound around 1100fps.)

Guns I have reviewed in this caliber include:

• PWS T3 Summit rifle
• Tactical Solutions TSG-22 Glock conversion
• Browning Buck Mark pistol

So this is a cheap, fun, and accurate caliber. But is it useful for hunting or defense? This is a subject of endless debate. When it comes to defense, of course, we would prefer to avoid confrontation altogether, and failing that would grab a high-powered rifle or shotgun to stop any aggressor. Smaller guns and lighter rounds are a compromise: you sacrifice power and penetration in order to get something more portable and shootable.

Effects on Humans

Common wisdom has it that .380ACP is “barely” enough bullet to qualify as a defensive handgun round, and anything lighter is more likely to enrage an aggressor than to stop him. My favorite study of this subject is An Alternate Look at Handgun Stopping Power by Greg Ellifritz. He analyzed nearly 1800 shootings during violent encounters and came up with some surprising results:

1. A lot of the time just shooting at someone is enough to get them to stop, regardless of caliber or whether they are hit. I.e., guns “psychologically stop” many assailants. Based on this observation: It’s more important to have a gun – any gun – than to be caught without one.
2. Determined aggressors do need to be “physically stopped” (incapacitated), and in that case shot placement is far more important than caliber. I.e., largely regardless of caliber: if you hit an assailant in the head they stop 75% of the time. Torso hits stop them 40% of the time. Put another way: How well you shoot is more important than what you shoot.
3. However, independent of shot placement, calibers below .380ACP are twice as likely to “fail to incapacitate” as the larger calibers. So yes, there is something to the conventional wisdom that if you’re carrying a gun it should shoot something no smaller than .380ACP.

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Interesting suggestion here that the NATO-standard 5.56x45mm infantry cartridge was selected based on short-range engagements that prevailed prior to the proliferation of rugged magnified optics. I.e., since it was hard to see and hit an enemy more than a couple hundred yards away, the 5.56mm cartridge was considered adequate.

Ever since its adoption field reports have complained about its marginal stopping power. In recent years our military has spent a lot more time in wide battlefields with optics good enough to reach out to and beyond the nominal 400-yard “effective range” of the 5.56mm. It may weigh twice as much, but nobody questions the ability of 7.62 NATO to stop human targets out to 800 yards. (Beyond that is the realm of trained snipers, who may step up to .300 Win Mag or .338 Lapua Mag cartridges with effective ranges beyond a mile.) There has also been a lot of recent work on other cartridges that fit within the 5.56x45mm profile but provide better ballistics and stopping power, 6.8SPC being the most widely adopted of that family.

Update: As further evidence of 5.56 inadequacy at longer distances: One range I frequent hung steel plates at 600 yards using fire hose. One day I noticed several .223 bullets stuck in the hose nose first: After 600 yards of flight they retained so little speed they stopped in the first layer of rubber!