A bullet has to spin at a certain minimal rate to maintain stability throughout its flight. Rifling imparts spin to bullets fired through a barrel, and it turns out that the twist rate of the rifling is somewhat particular to the bullets a gun is designed to shoot. We’re talking about rifles, and why are they rifled? In short: to keep the bullet pointed in the right direction. A 150gr bullet driven by the same load would be subsonic, but to be reasonably aerodynamic it would also be almost 2″ long, and this is where we run into some inconvenient facts about ballistics. 223 lead bullet, which shoots about 3000fps, is typically 3/4″ long. Let’s just double or triple the bullet weight,” one might naively say. This requires changing the rifle load so that the bullet’s velocity is reduced from its usual 2500-3200fps to a subsonic velocity - 1100fps under standard atmospheric conditions. Having absorbed most of the muzzle blast in the silencer itself the next step would be to eliminate the sonic crack. So at some point after one buys a rifle silencer one begins to wonder how to make it even quieter. In fact, people working in the target pits hundreds of yards downrange from a firing line are generally required to wear hearing protection because the sonic crack of bullets passing close overhead can still cause hearing damage. The largest component of the sound signature of a silenced rifle shot is the bullet’s “sonic crack”: A supersonic projectile creates pressure waves that reverberate along its entire flight path, and in the case of a rifle bullet these create roughly as much sound as an unsuppressed. A good silencer reduces that by 30-35 dB, making it hearing-safe to shoot, but by no means “silent,” nor even quiet. Why the obsession with subsonic bullets? The answer is, “Peace and quiet.” A typical center-fire rifle shot meters about 165 dB a few feet from the muzzle. To explain why that’s of any interest let’s take a few steps back. 22LR bullets significantly heavier than 40gr. The purpose of that post was to wonder why Aguila is the only manufacturer in the world making. 22LR Market, I figured it would be worth posting some of the notes I made during my last journey through that topic. Following some renewed interest in my old post on The Missing Subsonic.
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