What factors affect the volume of a bullet's sonic boom?

[Daekar]

I have been trying to do a bit of reading to determine this, but all the information I've found is concerned with aircraft. It appears that significant reduction in the magnitude of the pressure wave can be achieved by specially shaping the airframe, so it makes sense that some bullet designs might be louder than others, but without anything specific to go on it's hard to say whether it would be a difference you could actually detect. Bullets are just so small compared to aircraft.

Does anyone know if there are general rules we could use to calculate the pressure wave magnitude given a set of standard conditions? Cross-sectional area, meplat diameter, length, nose angle...?

I guess what I'm really trying to find out is if there is a meaningful subjective difference in the sonic crack between firing a suppressed supersonic 22LR VS something larger at an equivalent velocity under the same conditions.

[1Hawkeye]

It all depends on the projectiles speed anything moving higher than the speed of sound is going to crack rather than boom. Projectile shape has no effect a .45 cal round ball moving at 1600 fps is going to crack just like a .45 cal 300 gr hp moving at the same speed.

[cwtebay]

That's a very big question - bullet design does affect a projectile's performance passing transonic both directions, as well as the pressure wave that is produced. The airplane analogy is not far off - if you've ever been to an airshow or watched jets approach mach 1, you can see the pressure wave changing around the aircraft. There was a company doing quite a little research into design for both suppressed and non to mitigate the "crack" into something less acoustically objectionable.
That's been over a decade ago, so I would have to do a little digging to find that.

Sent from my Pixel 5 using Tapatalk

[~JM~]

VLD (Very Low Drag) Bullets?

Long gently tapered nose cone with a boat tail?

https://www.fieldandstream.com/blogs/the-gun-nuts/understanding-vld-bullets/

[BadgerShooter]

Judging from the sound passing overhead while working in the pits during highpower matches, I'd say the bigger the diameter, the louder the snap. I make a 30 cal 205 grain flat base low drag bullet and had lots of comments from pit workers when I was shooting that they were extremely loud going overhead. Noticeably louder than other bullets. My wildass theory is that the larger the diameter of the base of the bullet, the louder the snap, when the bullet goes past. I think boattails reduce this a bit by reducing the vacuum area behind the bullet. There is a noticeable difference between 223's coming over and 30 caliber bullets.

[~JM~]

https://www.grc.nasa.gov/www/k-12/airplane/short.html

https://www.grc.nasa.gov/WWW/K-12/rocket/shortr.html

[Larry Gibson]

With a given caliber it is the shape of the nose that actually affects at what velocity the bullet "cracks" at. In 30 cal for example, a more streamlined VLD BT'd bullet will sneak right on 1100 fps before a crack or snap can be heard right out in front of a good suppressor. Conversely, a WFN bullet with a flat base can begin cracki9ng or snapping as low as 950 fps. It is actually the speed of the air moving around the bullet that goes "sonic", not the speed of the bullet. If the VLD bullet cracks in front of the suppressor at 1100 fps and the WFN cracks at 950 fps that means the air is moving at or just above the speed of sound for both bullets.

The only "general rule" I know of regards the shape of the bullet is; the larger the diameter, the blunter the nose and the less tapered the base the lower the bullet velocity will be when the sonic barrier is reached.

[Wilderness]

With a given caliber it is the shape of the nose that actually affects at what velocity the bullet "cracks" at. In 30 cal for example, a more streamlined VLD BT'd bullet will sneak right on 1100 fps before a crack or snap can be heard right out in front of a good suppressor. Conversely, a WFN bullet with a flat base can begin cracki9ng or snapping as low as 950 fps. It is actually the speed of the air moving around the bullet that goes "sonic", not the speed of the bullet. If the VLD bullet cracks in front of the suppressor at 1100 fps and the WFN cracks at 950 fps that means the air is moving at or just above the speed of sound for both bullets.

The only "general rule" I know of regards the shape of the bullet is; the larger the diameter, the blunter the nose and the less tapered the base the lower the bullet velocity will be when the sonic barrier is reached.

Larry - Interesting stuff.

Next question: How then does this apply to bullets coming back DOWN to the speed of sound?

I'm thinking of those cast bullets in the 1200 to 1500 fps range that at some point between the muzzle and the target will have to come back through the sound barrier. Does a poor form, by keeping up the air speed around the bullet, mean that the bullet can be free of disruption right down to 1100 fps, or perhaps even lower?

Incidentally, I believe I see the effect of the transonic disturbance on the target as slightly elongated bullet holes, though it sometimes coexists with OK groups.

[gwpercle]

Are the Sonic Booms a cast boolit makes really a problem ?

The Sonic Booms in the late 50's early 60's sure did rattle our windows and moms nerves ... she even said bad words about our government ... but I don't remember boolits doing much .
Gary

[Larry Gibson]

Are the Sonic Booms a cast boolit makes really a problem ?

The Sonic Booms in the late 50's early 60's sure did rattle our windows and moms nerves ... she even said bad words about our government ... but I don't remember boolits doing much .
Gary

When shooting w/o a suppressor the muzzle blast masks the sonic boom sound. With a good suppressor the "crack" is heard just in front of the suppressor. It can be relatively loud or mild depending on the velocity, bullet shape and caliber. The sonic boom or "crack" also, if you are the shooter or by the gun, goes away from you following the sonic pressure wave as it follows the bullet. The sound also echo's off walls, trees, berms, rocks, buildings, etc. If you are under a covered firing position the echo off the roof and/or especially baffles can make the "crack" sound much louder than it really is.

Cast bullets, if sonic, do indeed "crack" just like any other sonic bullet.

Cast bullet sonic booms a problem? Only if you want a truly suppressed noise. Then the bullet must be sub-sonic on suppressor exit. The OP isn't apparently concerned about the sonic boom but the buffeting of the bullets as it transitions from sonic to sub-sonic.

[Larry Gibson]

Larry - Interesting stuff.

Next question: How then does this apply to bullets coming back DOWN to the speed of sound?

I'm thinking of those cast bullets in the 1200 to 1500 fps range that at some point between the muzzle and the target will have to come back through the sound barrier. Does a poor form, by keeping up the air speed around the bullet, mean that the bullet can be free of disruption right down to 1100 fps, or perhaps even lower?

Incidentally, I believe I see the effect of the transonic disturbance on the target as slightly elongated bullet holes, though it sometimes coexists with OK groups.

It is believed that blunt nose bullets with the center of gravity close to or ahead of the center of pressure make transition smoother with less disruption of accuracy or stability. There are numerous other things that can effect the fps range of that transition but it generally believed to be a +/- 200 fps range of buffeting from the actual sonic velocity. The "elongated bullet holes" you mention may or may not be from that transition buffeting. There are numerous other reasons for such. Hard to speculate w/o more details known of the load, the bullet, the velocity and specifics of the rifle twist.

You might shoot at 50 yards and at 150 or 200 yards. Round holes at those ranges or a total loss of accuracy at 150/200 yards would be a good indication the bullet is not making the transition well. It could also mean the bullet, if marginally stabile on muzzle exit is simply losing stability as the velocity decreases and the range increases.

[Daekar]

My primary question to myself was initially, "When my suppressor arrives, will it be worth developing supersonic loads for my 357mag to shoot through it?"

I find supersonic 22LR tolerable without hearing protection, and I was trying to find a way to determine how loud, comparatively, a 35 caliber RFN boolit will be compared to a 22 caliber HP when both are moving at low supersonic velocities. It is very interesting that the boolit shape makes that much of a difference in the velocity at which the air around the boolit can no longer remain subsonic while getting out of the way, and even more interesting to think that the bullet will be able to slow down more than a different design before suffering transonic instability.

[TurnipEaterDown]

Hopefully I understand the questions/conversation correctly, and so here is some comment:

A suppressor will deal with the pressure wave from the expanding combustion gasses.
Muzzle pressures are commonly somewhere 3-6K psi on centerfire rounds. Huge variety of CF rounds, barrel lengths, loads, so this is generalization. There are tools that will calculate this for you.

The expanding gas is a large component of the "Muzzle blast".
The bullet very much makes some of the noise, and as Larry noted, is affected by bullet size, shape, speed, but the very reason a suppressor makes a difference when shooting a bullet at supersonic speeds is that it can deal with the gas expansion.

A Suppressor will not deal with the pressure wave on a bullet in flight. It is in flight, and away from the muzzle with a strong pressure wave formed by the displaced volume of air from the supersonic bullet. A subsonic bullet will also displace air, and create localized pressure changes, but they do not create the same magnitudes of pressure differential in the wave of displaced gases.

As noted, some shapes are better at creating smoother pressure waves / disturbances in flight. This also plays into laminar flow, vs turbulent flow.

The pressure wave shape from the bullet flight is affected by bullet form.
Angle of departure of the shock wave from the bullet nose, etc. is affected by form/shape of the bullet.
There will also be some secondary waves from lube grooves, cannelure, base edge, etc. Any disruption in form will promote a change in airflow, and wave formation.

Pressure differential across the bow wave is predominantly affected by Mach number of the bullet in flight.
The pressure differential is the "strength" of the shockwave.

Pressure * Area = force.
A Higher pressure (stronger) wave impacting your eardrum makes a louder sound than a Lower pressure (weaker) wave.

Properly designed, and bullet's nose shape can & will have a strong effect on temporary and permanent wound cavity shape, and size, at least with bullets of supersonic flight speeds prior to impact. Even 14-1600 fps achievable with pistols w/ non expanding bullets can create notably different wound cavity character from sub 1000 fps impact.

Very much as previously alluded to, the turbulence of the displaced air is affected predominantly by shape of the bullet, and is key to stability in transition from supersonic to subsonic.

I remember reading model rocketry books (make your own) when I was a kid that discussed how center of pressure placement vs. center of gravity, and center of form is key to stability of a rocket (tailing into what Larry said in one post).
However, bullets and rockets are very different, so don't read that what applies to rockets directly applies to bullets. Rockets and arrows share stability mechanism: fins. Bullets are stabilized differently: Spin.
Loss of stability comes from an overturning moment created by an forces on the body (often at impact: creating yaw in body), or lack of concentricity of mass and form (weight off center of the body).

You really shouldn't have the presence of a suppressor affect the stability of the bullet in flight. If you know a particular bullet that you want to use, just load it to the same velocities for checking stability & groups at defined ranges.

Been years (30?) since I had to do math with supersonic gas flow, but hopefully my summary memories help some.

[shootinfox2]

The sonic crack/boom is the sonic shock wave separating from the actual body of the bullet, airplane, missile. The smoother the separation, the less crack. Early aircraft had an extended pitot tube in front of the aircraft to move the “wave” allowing later separation and drag. Swept wing aircraft were developed for the same reason. Allows the shock wave to shed rather than snap. You tube has many videos explaining how the shock wave develops and seperates.

[Geezer in NH]

When you want real quiet load subsonic. My mac10/9 and CAC by Bowers is Hollywood quiet with a 151 grn 38 super bullet and 6 grains of HS6. Bolt slap sound and big thump at a 50-yard bank.

Go sonic with a lighter bullet still don't need hearing guards but firing is noticeable. Who are you trying to hide the noise from? Any sound surpressor will hide location of the shot but anything the bullet goes past will crack in a sonic "Boom". Sound wise you hear it or you won't so who cares about the difference in bullet shape at that point. Just MHO

[samari46]

Living on Long Island,NY one always knew when the French jet with the drop down nose was coming into either La Guardia or the other major airport. The jet would generate a sonic boom just as it was passing over our neighborhood in Elmont,Long Island. Can't remember the name of the jet. Frank

[M-Tecs]

That would be the Concorde. Sonic booms are continuous. As long as the item is above Mach 1 it produces a continuous sonic boom.

[405grain]

Something that I've thought about: When you push an object through a fluid, (air), faster than the molecules can move out of the way, the air ahead of the object is compressed into a shock wave. Inside this shock wave the object is moving in a low pressure region, (a partial vacuum). The reason that a bullet will decelerate rather quickly, but it's rate of rotation slows very little, is because it is compressing all that air ahead of it while the bullet is spinning in a near vacuum. What I've wondered about is when a bullet passes through a point in space while traveling at supersonic speed, after it has gone beyond that point the compressed air, (and air in general), will within a short period in time collapse back into the partial vacuum created by the bullet. When that happens it surely must add to the crack sound as the air slaps back together.

[M-Tecs]

I know size matters. I was about 50 yards under two supersonic F-15's that had just scrambled on an alert. Loud is an understatement.

[DonHowe]

Contrary to what may shooters seem to believe, the same physics and aerodynamics that apply to airplanes and rockets apply to our bullets. That most of the time the science is of little consequence to our fun does not negate the scientific facts.

[Larry Gibson]

Contrary to what may shooters seem to believe, the same physics and aerodynamics that apply to airplanes and rockets apply to our bullets. That most of the time the science is of little consequence to our fun does not negate the scientific facts.

^^^^What DonHowe said^^^^^^

[El Bibliotecario]

I recall reading that prior to WW I the army issued Maxim silencers--I want to say two per rifle company. Apparently the program was abandoned when they discovered the facts as discussed above. Firing the M1903 rifle parallel to a railroad line produced negligible noise from the weapon, but a staccato CRACK! CRACK! CRACK! CRACK! CRACK! as the bullet passed the telegraph poles.

[charlie b]

Yep.

I have experimented a bit with this using light loads in the .308, and some longer distance shooting. I have three bullet shapes. The flat nose cone shape of the 210gn Eagan MX3, the pointed 165bn XCB and a rounded nose of the 180gn Lee and others.

My light loads are 1000-1200fps. As Larry mentioned, the transition zone covers about 200fps around the speed of sound, which here is around 1090fps (altitude). The XCB and Lee shoot very well at these velocities. When fired at 1100fps I can hear the crack. When fired at 1000fps or lower I cannot. Theory says that the shock wave formation and 'movement' are 'gentle' with rounded profiles and pointed tips.

I shoot the MX3 at longer ranges. With a MV of 1800fps it is only going ~1050fps (or less) at 500yd. The bullet is yawing at that point. If I increase the velocity to 2000fps and it is ~1200fps and retains the accuracy and nice round holes in target.

My theory is the flat tip and cone section of the MX3 cause a 'jump' as the shock wave moves from the shoulder of the cone to the flat nose, which disrupts the bullet stability.

So, yes, shape is important.

Noise level is two parts. Flat bases are larger in dia meaning more intense shock. Boat tails basically have two shocks, with one from the smaller dia base, so the overall effect is lower.

[Larry Gibson]

I recall reading that prior to WW I the army issued Maxim silencers--I want to say two per rifle company. Apparently the program was abandoned when they discovered the facts as discussed above. Firing the M1903 rifle parallel to a railroad line produced negligible noise from the weapon, but a staccato CRACK! CRACK! CRACK! CRACK! CRACK! as the bullet passed the telegraph poles.

Yes, there is an echo off of objects down range. What they observed and found objectionable was from the shooters position. What they failed to do is observe what is heard, or not heard, from the target end of the test. Had they done that they would have observed how confusing those echos are to other enemy soldiers. The intended target should not be aware of the echos or anything else. The objective of the use of a suppressor is to conceal the location of the shooter by suppressing the muzzle blast. If used correctly it does that. Additional benefits are the shooter isn't affected by the muzzle blast, the suppressor is also a very effective flash suppressor and the suppressor often improves accuracy because it can dampen barrel vibrations.

Having thoroughly tested numerous suppressors, including maxims, from both ends I can say for certainty that suppressors are very worthwhile in combat situations. If you are in the target area [basically a 120 degree cone out from the shooter] it is very difficult to tell the direction the shot came from. When you hear the "crack" and a buddy crumples and everyone dives for cover the problem becomes, since there was no "bang" heard to locate the direction the shot came from, are you behind cover or in front of cover...... that situation can be very disconserting.....

[charlie b]

Looked up a couple of videos.

This one is pretty good and shows the bow and tail shock waves, as well as a comparison between supersonic and one just below, or at the beginning of transonic.
https://www.youtube.com/watch?v=BPwdlEgLn5Q

This article has some pics that show transonic changes.
http://bulletin.accurateshooter.com/2014/09/practical-thoughts-about-transonic-bullet-stability-and-accuracy/

And here is a blunt nose/flat base bullet.
https://www.nasa.gov/images/content/185590main_F-Bullet-Shock.jpg

Also a book called "Modern Advancements in Long Range Shooting" is available on Kindle Unlimited. Browsing through it will give any answers you need. Also includes a table of different bullet profiles and data on them.

At one time I saw a wind tunnel test that showed transition from subsonic to supersonic and the movement of the shock waves over the length of the bullet. It was enlightening.

[samari46]

M-Tecs, thanks for jogging my old brain cells. That Concorde would really rattle the windows. Of course the flight path would change due to weather and other factors. Was on an aircraft carrier during the Vietnam War. Those little Skyhawks would really rattle your eyeballs especially if they had a heavy ordnance load out and took off with their afterburners lit off. Watch air ops, then head back to my nice less noisy engineroom. Frank

[charlie b]

Found another nice one. Sherman tank with high speed video. Go to the 8:10 point. Shows several things. Shock waves on 'bullet', 'wobble', spin. I think they said 2500fps.

https://www.youtube.com/watch?v=xpJ8EoGmLuE

Bonus: At the end they fire a howitzer into a line of watermelons. Spoiler: the watermelons actually deflect the shell!!!

[Shiloh]

Humidity, barometric pressure, and air temperature, will adjust the speed of sound.
Anything faster than 1125 fps, adjusted for the variables listed, is supersonic.

Shiloh