Bullet 'crack' at subsonic velocities?

[Short-bus]

Ok, I just got back from shooting a bit ago and was reviewing target cam footage from 1180 yards. I'm shooting 175smk's at 2650, conditions were 29.22" 48% RH and 45 degrees.

There is quite a bit of noise as the bullet goes by, however it's not the high pitched crack that you get when shooting a very much supersonic bullet, it's a bit lower sound. At first I thought it was the muzzle blast, but when turning up the speakers, I can hear the muzzle blast. Am I hearing the supersonic crack from the bullet 80 to 100 yards earlier after it dropped subsonic? Or what am I hearing?

I'm going to edit the video to include clips of misses so that only the crack is heard, and not the steel getting hit. I wanted to get some opinions here. It would be great to get Brian Litz's, or Lindy's thoughts on this. I'll get the video put together ASAP and update. Until then, opinions?

Branden

Update: Here's the video, I rushed it so it's not very good.

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[LRS_Ranger]

Re: Bullet 'crack' at subsonic velocities?

Interesting thought. I noticed this once while I was out marking the target for a buddy. You can't tell from the cam, but my experience was hearing a less intense crack, seeming to originate from in front of the target. When the round is still over the speed of sound, the sound seems to come from 90 degrees to the path of the bullet. I can only assume that you are hearing the sonic crack from before the bullet went transonic. I was at 1K and the bullet was GMM 168's, so I am guessing they were dropping below Mach 1 roughly 50-75 meters in front of the target, which is roughly where I heard the sound coming from.

 

[sandwarrior]

Re: Bullet 'crack' at subsonic velocities?

Try listening to a .22 bullet at around 200 yds. You can still hear it pretty well even if by then it is subsonic.

 

[hankpac]

Re: Bullet 'crack' at subsonic velocities?

While target setting at the international silhouette shoot in Texas, I heard subsonic large bore rounds going down range. I was standing at the side of the range, about half way down, 100 yards from the edge and behind a barrier. Most shots went Boom, and we never noticed a supersonic crack, but the subsonic guys, would make a long whooshing sound all the way from the line to the target (if I can duplicate it in writing) sort of a Boom, shshshshshshshshshshshshshshshshshshshshshshshclang. You could follow the path with your head, it was so slow, and the swishing sound was distinctive.
In firefights, AK and SKS rounds would snap overhead, Very sharp, not explosive like the noise on the video above, but more of a SNAP. Very short and very sharp. I have duplicated that in the range when siting behind a berm, setting targets for .223 at 2900 fps (muzzle) at 300 yds.
We watched these guys shoot all day for 5 days, so there was no mistaking when the subsonics came to the line.

 

[Short-bus]

Re: Bullet 'crack' at subsonic velocities?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: hankpac</div><div class="ubbcode-body">While target setting at the international silhouette shoot in Texas, I heard subsonic large bore rounds going down range. I was standing at the side of the range, about half way down, 100 yards from the edge and behind a barrier. Most shots went Boom, and we never noticed a supersonic crack, but the subsonic guys, would make a long whooshing sound all the way from the line to the target (if I can duplicate it in writing) sort of a Boom, shshshshshshshshshshshshshshshshshshshshshshshclang. You could follow the path with your head, it was so slow, and the swishing sound was distinctive.
In firefights, AK and SKS rounds would snap overhead, Very sharp, not explosive like the noise on the video above, but more of a SNAP. Very short and very sharp. I have duplicated that in the range when siting behind a berm, setting targets for .223 at 2900 fps (muzzle) at 300 yds.
We watched these guys shoot all day for 5 days, so there was no mistaking when the subsonics came to the line. </div></div>

When I was firing from the 1180 line, it did sound like I could hear the bullet going downrange. Bang.hisssssssssssssssssssss return echo of the blast. It's a good 5 or 6 seconds before the echo returns back to me, it's something else!

Branden

 

[flyboy]

Re: Bullet 'crack' at subsonic velocities?

I'm not sure about bullets but the theory should hold. In aircraft or as I understand it, a supersonic shock wave develops on the object moving in the transonic regime of flight. Before the X1 broke the sound barrier, but was close, observers reported a boom from the overflight of the aircraft. Flights in the supersonic regime came with a "BA-BOOM" or double bang.

For an example the jet I fly begins to develop supersonic flow over parts of the aircraft as low as .65 Mach.

Again, I'm just speculating.

 

[Augustus]

Re: Bullet 'crack' at subsonic velocities?

I too would like to know the answer to this question because I am using this method to determine when the .375 6.5 cal. ZA 400 gr. goes sub-sonic. So far it has made the loud snapping crack out to 2560 yds. That is as far as I have taken it to date.

 

[groper]

Re: Bullet 'crack' at subsonic velocities?

Flyboys assesment is spot on... you can get supersonic flows over parts of the body in flight, even tho the body itself ISNT supersonic... that and the fact that the 'crack' is quite loud, means that it would be easily audible from 80yds away...

Why dont you take it out a bit further say another 100-200yds, then repeat your test again...

 

[Augustus]

Re: Bullet 'crack' at subsonic velocities?

I intend to do that very soon. I am waiting on more boolets.

 

[Augustus]

Re: Bullet 'crack' at subsonic velocities?

Actually this bullets has progressed to the point it really needs to be radar tested to find out its true BC. The transition range needs to be identified. Also, I would like to know if it will transition without going bezerk.

 

[Short-bus]

Re: Bullet 'crack' at subsonic velocities?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: groper</div><div class="ubbcode-body">Flyboys assesment is spot on... you can get supersonic flows over parts of the body in flight, even tho the body itself ISNT supersonic... that and the fact that the 'crack' is quite loud, means that it would be easily audible from 80yds away...

Why dont you take it out a bit further say another 100-200yds, then repeat your test again... </div></div>

Good idea, i'll give it a shot. I only have another 80 yards that I can move back in that spot before i'll be breaking the law of no shooting within 100y of a public road (questionable since it's a gravel road, but why risk it). I may look at angling my shot to get somewhere around 1400 and try again. We'll see if the winds are calm enough over the Christmas break. I suppose I can always drop the powder charge and trim off 150fps with the sole intention of just listening to the bullet pass by.

Anyone else notice the tone change of the crack between the 700 and 1008 yard shots?

Branden

 

[Short-bus]

Re: Bullet 'crack' at subsonic velocities?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: groper</div><div class="ubbcode-body">that and the fact that the 'crack' is quite loud, means that it would be easily audible from 80yds away...</div></div>

I went in and ran the number through JBM using Litz's G7 BC's and found that the dope matched up with my results from the field today. Using that info, the bullet went subsonic or at least started the transition at 1023 yards, ~150 yard prior to the target.

Branden

 

[flyboy]

Re: Bullet 'crack' at subsonic velocities?

Instead of looking for more distance to test the theory, you could always slow the bullet down and shoot at a moderate distance of 1000. Say make the bullets fly at 2300 MV. It would either prove or disprove the ideas above.

 

[BryanLitz]

Re: Bullet 'crack' at subsonic velocities?

I agree with flyboy and groper. The transonic zone is classically defined as Mach 1.2 down to 0.8, but that depends on the shape of the object. The actual speed of sound is the peak of transonic effects, but it's not the only speed at which transonic 'messyness' applies.

I doubt the bullet is below Mach .8 at the target (.8 times the speed of sound) so you probably have supersonic flow compressing air into shockwaves at some point on the bullet. It won't be as complete or strong of a shock as when the bullet is fully supersonic, (that's why the crack sounds different) but if there's supersonic flow over any part of the bullet there will be a 'crack'.

There's also the sound from behind the bullet that you will hear when the shock was stronger and louder. I'm pretty sure that's the ssssshhhhhhhhh..... you hear when shooting long range is the shock wave emanating from the egressing bullet.

-Bryan

 

[Short-bus]

Re: Bullet 'crack' at subsonic velocities?

Brian,

Thank you for taking the time to provide us with your input on the matter. All this is very enlightening for me, and very new.

Branden

 

[Victor Co Heather]

Re: Bullet 'crack' at subsonic velocities?

Great post, thanks for the info! Working the pits, I've heard the difference in supersonic and subsonic bullets. While they both made noise, the subs were much more comfortable noise-wise to the ears.

 

[bohem]

Re: Bullet 'crack' at subsonic velocities?

Flyboy's hunch is exactly right. Ply1951Guy and I have done some CFD work as we announced in another thread, but we haven't limited it to supersonic bullets.

In subsonic bullets we suspected the localized flow over the bullet was breaking the sound barrier and the analysis confirmed out hunch.

If you keep a RN bullet under about 975fps and the spire-pointed things like say a 220 SMK under about 990fps the localized flow will stay completely subsonic. This jives with the real world testing that I've done on subsonic ammo and noticed a large difference in sound signature once the bullets dropped below about 975-ish FPS muzzle velocity.

ETA: This is for STP conditions, if it's very cold out (or the DA is very low) then the speeds go down more.

 

[flyboy]

Re: Bullet 'crack' at subsonic velocities?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body">I agree with flyboy

-Bryan </div></div>

I just had to!!!

 

[Fred_C_Dobbs]

Re: Bullet 'crack' at subsonic velocities?

The velocity at which localized flow breaks the sound barrier is known as the "critical Mach number."

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bohem</div><div class="ubbcode-body">...If you keep a RN bullet under about 975fps and the spire-pointed things like say a 220 SMK under about 990fps the localized flow will stay completely subsonic. This jives with the real world testing that I've done on subsonic ammo and noticed a large difference in sound signature once the bullets dropped below about 975-ish FPS muzzle velocity..... </div></div>
I've chronographed round nosed Aguila 60-gr .22LR SSS in the low 900 fps range from a 16" barrel. That they gave SoS such a wide berth would seem to indicate there's more to it than meets the <span style="text-decoration: line-through">eye</span> ...uh ...ear.

 

[SPAK]

Re: Bullet 'crack' at subsonic velocities?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bohem</div><div class="ubbcode-body">Flyboy's hunch is exactly right. Ply1951Guy and I have done some CFD work as we announced in another thread, but we haven't limited it to supersonic bullets.

In subsonic bullets we suspected the localized flow over the bullet was breaking the sound barrier and the analysis confirmed out hunch.

If you keep a RN bullet under about 975fps and the spire-pointed things like say a 220 SMK under about 990fps the localized flow will stay completely subsonic. This jives with the real world testing that I've done on subsonic ammo and noticed a large difference in sound signature once the bullets dropped below about 975-ish FPS muzzle velocity.


ETA: This is for STP conditions, if it's very cold out (or the DA is very low) then the speeds go down more.

</div></div>

I've noticed this as well that as temperature drops so does the velocity needed to break the sound barrier. My same loads that run 1090fps in the summer that are subsonic have a very audible sonic crack at colder temperatures.

 

[bohem]

Re: Bullet 'crack' at subsonic velocities?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fred_C_Dobbs</div><div class="ubbcode-body">The velocity at which localized flow breaks the sound barrier is known as the <span style="font-weight: bold">"critical Mach number." </span>


I've chronographed round nosed Aguila 60-gr .22LR SSS in the low 900 fps range from a 16" barrel. That they gave SoS such a wide berth would seem to indicate there's more to it than meets the <span style="text-decoration: line-through">eye</span> ...uh ...ear. </div></div>

Exactly right on the CMN. The 60gr Aguila SSS ammo seems to be somewhat noisy from our testing, I have a feeling it has less to do with the nose and a lot to do with the tail of the bullet. The ammo is heavily crimped with a hollow base, I have no idea what it looks like when it leaves the barrel, so it's very hard to make an accurate CAD model of it. The nose geometry on that stubby little thing might be just dandy, but the tail could very easily be screwing things up instead.

With a jacketed rifle bullet or a gas checked cast bullet the geometry is much more predictable and I have more confidence in the analysis that we did. To be totally truthful I'm not sure how I would get an idea of what the 60gr SSS ammo looks like in flight without spending a bunch of money in high speed video. I haven't seen one of those bullets yet in the footage on youtube. If you come across it please let me know, we'll look at it for sure.

 

[Fred_C_Dobbs]

Re: Bullet 'crack' at subsonic velocities?

In the same vein, does the tip of a bullwhip really break the sound barrier to make its signature crack or are the aerodynamics of a strip of leather so abysmal that its critical Mach number is ...like ...1/2 Mach.

 

[JL]

Re: Bullet 'crack' at subsonic velocities?

 

[sandwarrior]

Re: Bullet 'crack' at subsonic velocities?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JL</div><div class="ubbcode-body">

</div></div>

That's an interesting graph. Although, I wasn't there to measure it I don't agree with it. I've stood in the pits many times on shorter ranges where the bullets were still well above supersonic and they didn't make 140 db's. I've worked on more than a few aircraft ramps and I'll tell you a APU only makes 140 db and it is LOUD. I will say a supersonic bullet crack is louder than a sub-sonic bullet going downrange. So, the shape of the graph is right, just not the numbers.

 

[JL]

Re: Bullet 'crack' at subsonic velocities?

dB graph is correct.

Without going into details, comparing desibels vs practical felt loudness is not that simple.
dB is pressure, but moved air volume makes big difference in practical noise level.
Small flute and big motorcycle can produce same decibels.

Pistol shrimp snaps claws and makes 218dB pressure and jet engine only 150dB or so, but felt difference is still big for same reason.

 

[sandwarrior]

Re: Bullet 'crack' at subsonic velocities?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JL</div><div class="ubbcode-body">dB graph is correct.

Without going into details, comparing desibels vs practical felt loudness is not that simple.
dB is pressure, but moved air volume makes big difference in practical noise level.
Small flute and big motorcycle can produce same decibels.

Pistol shrimp snaps claws and makes 218dB pressure and jet engine only 150dB or so, but felt difference is still big for same reason.
</div></div>

That makes sense. It just never seems like it was that loud. Probably because I wasn't that close.

So, question on that then? Using your explanation, how far away from the bullets path in std. air density (29.92/59.7 deg.) would you hear that snap? Typically, to me anyways, we could be about 20 yds off to one side and hear a very distinct 'snap', but nothing in the realm of painful to the ears.

Point being, a shooter goes into an x-populated area and wants to know if his weapon of choice/suppressor is going to be worth having that day. Or he need make a change to increase effectiveness.
Edit: The above would be a scenario from Afghanistan.

 

[John M]

Re: Bullet 'crack' at subsonic velocities?

This is a very interesting topic. I have very little to add, certainly from a scientific perspective.

I shoot the Aguila Match Rifle 22 ammo (40 gr). On the box, it says 1080 fps and I do understand that altitude and temperature affect speeds.

I do know for a fact that standard velocity 22 ammo is significantly louder than the subsonic stuff I shoot (in fact hunt squirrels with).

I have had several people ask me if I had a misfire!

I just tell them to ask the squirrel lying at the bottom of the tree. Without question the subsonic ammo shoots truer in my setup. But there is a marked difference in the sound even to the untrained ear between supersonic and subsonic velocities in 22 ammo.

Nice info guys, keep it coming!