In Part I we noted that water provides a good model of a bullet’s terminal ballistics. We have also spent some time with our water facility and the high-speed imaging technicians from Aimed Research studying the dynamics of ricochet.

Following extensive experiments we have observed that pointed and round-nosed bullets will almost certainly ricochet out of the water when fired at incident angles flatter than 10 degrees. Even superstabilized bullets leave contact with the water tumbling erratically, and give up as much as 90% of their energy to the impact.

Aimed Research provides high-speed video of the behavior. This is a 225gr .30 caliber bullet:

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This is my recent AR-15 pistol build. The upper assembly from CMMG was $700 and features a KeyMod free-float handguard and 8″ medium barrel. It is chambered in .300BLK, which is currently the most versatile and efficient AR-15 caliber for such short barrels. I assembled the lower from a Seekins Precision forged receiver, Phase 5 Tactical pistol buffer tube, Noveske QD end plate, CMC 3.5lb single-stage trigger, and Stark SE-2 grip. Total component cost for the lower was $450. Unloaded weight for the complete firearm is only five pounds.

A generation ago sub-machine guns were the middle ground between handguns and full-power rifles. A famous photo from moments after the 1981 assassination attempt on President Reagan shows a Secret Service agent readying an Uzi he had produced from a nearby briefcase. We have since learned that power is more important than volume of fire, and that guns on full-auto with concealable magazines run empty far too quickly. Today the niche between handguns and long guns is filled with rifle-caliber “personal defense weapons” (PDWs), which are powerful enough for a serious gun fight but still portable enough for every day use and potential concealment.

The advantage of a “pistol” AR-15 is that it can be equipped with a barrel shorter than 16″ without the hassles of registering it as a Short-Barreled Rifle (SBR). What qualifies it as a pistol is that it doesn’t have a full stock or second vertical grip, yet the buffer tube required for the AR-15 to function can serve in a pinch as a three- or four-point mount, providing accuracy on par with a traditional carbine. (The ATF, struggling to make 80-year-old laws look reasonable in spite of evolutions in gun design and tactics, recently ruled that even arm braces do not turn a pistol into an SBR.)
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A few months ago I discussed metals and coatings for firearm actions. I noted the NiB (nickel-boron) gets discolored by fouling, but my photos only showed a sparkling new NiB-X BCG. Following is a picture of what it looks like after a few hundred rounds of use, followed by ultrasonic cleaning and then aggressive scrubbing with steel and brass wire brushes. For comparison I show my heavily-used chromed BCG on top.

Is this just a cosmetic issue? This is the only cleaning I’ve given the NiB BCG. I haven’t lubed it and I have subsequently run a few hundred rounds more without any action failures. However it seems plausible that if fouling can bind to the surface this stubbornly it could build up to the point of overtaking the nickel-boron’s lubricity and causing a stoppage that only traditional lubricants prevent. As noted in the original article this is not a problem with chrome and NP3: All photos of those to date have been after they were used and wiped clean with minimal brushing.

Update: A number of people say it’s unfair to compare a BCG from a piston gun to one from a DI gun, since the latter is subject to much harsher fouling. So for comparison I pulled and cleaned an NP3-coated BCG I’ve been running in a DI gun: pictures in my comment below.

This sequence shows me discharging a loaded round I couldn’t disassemble. There is no reason to ever do something like this other than brazen curiosity. If you want to disable a live round you should pull the bullet and dump the powder. If for some reason that fails a safer alternative to discharging it is to “cook it off” in an open pit fire. (Ensure that anyone not wearing a face shield and thick clothing stands clear until it pops.)

Firearm cartridges are not particularly powerful or dangerous unless they are tightly confined. Without a gun barrel to contain and direct the pressure smokeless powder burns slowly, if at all, and bullets are propelled only by the force of the primer. (Granted, primers are not toys. They are true explosives. Small firearm primers produce 5-10 foot-pounds of energy, and can produce pressures on the order of 25kpsi in a small closed chamber. Like firecrackers, they can burn and maim.)

The round I had on my hands contained a full load of powder that turned out to be too fast for the bullet. I managed to pull the rest of the batch, but one bullet came out and left its copper gas check in the case. In that condition it could have been safely fired in a gun, except that it could have badly fouled the bore depending on how the gas check engaged it. So instead I drilled a hole in a piece of wood to tightly hold the case neck, put on leather and a face shield, then detonated the primer with a steel punch.

An integral pulled bullet and case are shown left. The hand-fired case and bullet missing its gas check are on the right.

The problem with any containment when discharging a round is that without experience and knowledge of the case and powder you may be surprised at where the force ends up. The unsupported case could become a projectile or fail and produce shrapnel. The bullet and any other particles in the path of the venting gases can also be ejected almost anywhere. The setup above was carefully planned to allow for the worst possible outcome in every dimension. What actually happened is that the case neck held fast in its hole in the upper plank and the unsupported annealed upper body was blown out by the pressure, but did not fail. The gas check ended up embedded in the bottom plank directly below, and the gas was able to vent out the gap between the planks, blowing only minor wooden debris along with it.

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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