https://archive.ph/rMK5M#selection-175.0-177.23
There are thousands of indoor ranges in the United States, and many have a similar layout: A row of lanes separated by bullet-resistant walls. Soft, shock-absorbent paneling is not standard and without it the booth can act like an echo chamber, reflecting more of the blast waves back toward the shooter.
The U.S. military currently says that any blast wave that peaks below 4 P.S.I. is safe — though that guideline is not based on solid evidence, and will likely change as research progresses.
In The Times tests, sensors showed that the booth walls doubled the peak pressure for many guns, compared with shooting in the open. With a .357 revolver, the peak pressure tripled.
The smaller-caliber weapons that The Times tested at an indoor range created blasts that measured 1.3 P.S.I. on average — far below the military’s 4 p.s.i safety threshold.
The potential for harm from blasts is so poorly understood that even the U.S. military’s official safety threshold — 4 P.S.I. — is not based on real evidence. It’s only a place-holder, borrowed from decades-old guidelines for eardrum injuries. The military uses it because it doesn’t have the data yet to arrive at an evidence-based number for the brain.
The actual threshold for brain tissue damage may be much lower than 4 P.S.I. The Canadian military’s recommended safety threshold is 3 P.S.I. A U.S. Army study found symptoms of brain injury in grenade range instructors who were exposed to hundreds of blasts that measured less than 1 P.S.I.
It’s not clear how low the injury threshold might go. It’s not even clear that measuring peak pressure is the best way to gauge the risk.
The current military safety guideline looks only at a blast wave’s highest overpressure peak. But blast waves have dozens of peaks that can pulse through the brain in a few milliseconds.
Increasingly, Army researchers believe that it may be just as important to calculate the total area of all those peaks — a measurement known as “positive impulse” — and then add up those measurements for every blast a person is exposed to in a day.
In recent studies, the Army found that soldiers who were repeatedly exposed over a day of training to a number of blasts that measured around 4 P.S.I. were likely to show signs of slowed cognitive functioning — a possible indicator of brain injury — if the cumulative positive impulse of those blasts added up to 31.7 P.S.I.·milliseconds or more. In other words, multiple blasts within safe levels appeared to add up to an overall exposure that was not safe.
Based on the findings, the military is now considering whether to use a cumulative positive impulse exposure of 31.7 P.S.I.·milliseconds as part of a new daily safety limit. The potential new approach marks a stark change in how the military thinks about the hazard of blast waves. It is being reported here for the first time.
The Times measured an AR-15 rifle against that limit and found that while the peak overpressure of one shot was just 1.6 P.S.I., it took only 20 shots at an indoor range to exceed the proposed daily threshold for cumulative positive impulse. And it can happen fast.
Discussions in online shooting forums show that shooting enthusiasts regularly mention concussion-like symptoms, but talk about the risk of brain injury is rare. Post-shooting symptoms like headaches, fatigue and brain fog, which may be related to a brain injury, are often attributed instead to noise, tight-fitting protective gear, dehydration or poor ventilation.
Fortunately, there are simple ways to limit exposure. The Times found that shooting in an open outdoor setting, rather than in an enclosed booth, can cut blast levels by more than half. When shooting indoors, ensuring that the gun barrel extends beyond the booth can reduce exposure. Choosing smaller caliber weapons and less powerful ammunition can also help.
Attaching a suppressor or blast regulator to the muzzle to direct the blast forward and away from the shooter can also make a big difference. In The Times testing, the blast from firing an AR-15 rifle indoors measured as high as 1.7 P.S.I. When a blast regulator was added, the measurement fell to less than 0.5 P.S.I.
Suppressors and regulators are often used by law enforcement and the military, but they’re pricey, and suppressors are tightly regulated in some states and illegal in others.