9mm out of a 4" barrel and .223 out of a 16"-18" barrel are probably both running around the same pressure at that point
I agree they are probably roughly comparable (minute of ear-splitting
). Unbraked AR with a full length barrel and a shortish 9mm? But apples to apples, we are talking PDWs/Carbines here, so for the slightly longer barrel of a PCC (usually 5-6") and the shorter barrel of an M4 or M4gery (14.5" or ~15.5" + flash hider, right?), the sound/blast levels will differentiate much more in practice. With a 10" AR "pistol," it's not even close (I've never heard a Glock set off car alarms, but a PLR at the local range does it with regularity
).
16" in a 9mm is totally excessive (velocity is actually lost by going long enough to be legal), so I wonder if pressures aren't nearly ambient by the time the bullet squeaks out? Such a long barrel basically acts like an expansion chamber/linear comp, or even a silencer if you go long enough
I'm not sure what bearing this has on guns without comps or brakes?
That's my bad; I meant to say "or how the sound would
not be omnidirectional"
. What I mean is that in an interior setting, these loud noises will surround and engulf us so much that their directionality in an unrestricted setting will be moot. The obvious solution to the conundrum is to not force yourself to take a shot in a tight space or corridor, but whatever
Thread jack on muzzle devices (and their ability to make a loud gun into a quiet gun
)
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Overpressure doesn't behave like a normal pressurized gas stream, which tends to follow the course its momentum carries it on, fanning out into areas of lower pressure while decelerating. Overpressure is supersonic, and will expand in a roughly spherical bubble from wherever it is vented, the surface of which is a strong shockwave traveling faster than the speed of sound (detonation wave). As the pressure quickly equalizes from its expansion, the shockwave slows to the speed of sound and becomes a non-concussive sound wave. Muzzle devices can control the angle at which the bubble begins expanding relative to the bore, and in so doing direct the gas's overall momentum. But that bubble will still expand spherically and direct strong sound waves at the shooter (I don't think pistol or even 223-class rifle rounds have a big enough overpressure wave to engulf the shooter themselves
).
(For some reason, I'm thinking this famous old photo is of a 30-06
)
I've always understood 'brakes' to vent the overpressure backward (at an angle) so it's slightly louder for the shooter, and gawdawful for anyone off to his sides
--but recoil is reduced. The muzzle energy expands more in the direction of the shooter/bystanders, and arrives sooner, and therefore stronger.
Comps vent the overpressure upward or radially to hold down the muzzle, and are marginally louder to the shooter (not as much since the angle of blast is further forward, though), unless there is a steel roof overhead, at which point you receive the blast minus some loss from the reflection*. I associate comps with designs meant to combat muzzle rise rather than recoil. The sound still reaches the shooter, but the bubble can expand more --and dissipate more energy-- before it hits them.
"Cup type" linear comps essentially place a "blast shield" between the expanding overpressure and the shooter, focusing its energy forward. I've never quite understood the physics of how amplifying recoil force rearward keeps the muzzle down (unless your linear comp has a bias to the shape of its opening like any other vertical compensator), but the blast expansion is both directed entirely forward, and moved further from the shooter by the length of the comp. By the time the soundwave bubble hits the shooter, it has expanded as much as it can, and has lost the most energy we hear/feel as muzzle blast.
You also have linear comps which are basically silencers without baffles (I think) and holes punched in the forward side. Just like silencers, they allow the gas to expand inside a contained environment, where its energy is reflected and dissipated as turbulence, while slowly let out the smaller openings at the muzzle. Like silencers, they do a job of internally handling gas pressure, much as a locked breech does bolt thrust. However, they add length to the firearm, and are limited in blast reduction by their volumetric capacity. Not being silencers, they can't appreciably reduce sound levels, either.
All compensators that employ an expansion chamber will increase the duration of pressure on the bolt face, relative to the size of the chamber. Locked breech ARs can handle this additional duration better than marginal-bolt-weight blowbacks (though most BB bolts are heavier than needed). A device I'd like to see, if the ATF would have it, would be an integral linear compensator; port the barrel along its length into a chamber that is vented or entirely open at the muzzle end (and probably along its length to meet ATF regs). Properly designed, the overpressure bubbles would disrupt each other inside the tube and delay their propagation toward the exit, reducing felt overpressure and recoil with no additional barrel length.
Please correct me on any of this; it's merely a summary of how I've come to understand how these things to function. I'm always eager to learn more about this field (I'm an aerospace engineer, and high speed fluids is both a complex and unintuitive science)
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*Sheetrock is an excellent acoustic insulator/absorber --that's why we use it. A gunshot would probably be silent after passing through or reflecting off a mere handful of layers. But in a hallway setting like I describe, the single reflection won't be enough to even take the edge off a sound/shockwave, to say nothing of the overpressure (if the rifle is powerful enough for that to reach you)
TCB
. Although, that weight does lend good stability (until the shaking sets in).
