Supersonic bullet sound.

When I fire my rifle I can hear a “snap” immediately after the “bang” of the round. At first I thought it was the bullet hitting the tatget, but then I fired past it and still heard the snap.
I’m wondering it this sound could be the bullet breaking the sound barrier. I think that the compression(?) wave has to pass the listener for him/her to head the boom, but I’m not sure how it works.

Actually, I have no desire to “head” a boom, but to hear one would be okay.

I’d like an authoritative answer to this too. My WAG is that it is a reflection of the shockwave which accounts for the perceptable delay but I hear hear the distinctive crack even when firing handguns in open areas with not much to reflect the sound back to me.

Though we usually think of a “sonic boom” as a forward moving cone of shock and noise, there’s no reason to believe it doesn’t radiate backwards, too. If you stand at the target range and yell downrange, a person standing behind you can hear you before the echo comes back.

My ‘first job’ was pulling targets at a two-day rifle competition. I was amazed how loud it was. Even with the shooters 1,000 yards away, the noise was deafening. Someone told me that I was hearing the shock wave of the bullet as it passed overhead.

Yes, but is that cone that produces the percussive sound of a sonic boom. It seems to me that standing within the cone the sound should be no different than that of a subsonic bullet. I’ve compared this sound with handguns since there are both subsonic and supersonic ammunition available and there is a distinct difference in sound.

Typical rifles have much more muzzle blast which makes it harder to distinguish the supersonic crack but I had a chance to listen to a sound suppressed .308 rifle once. Even with standard supersonic ammunition the suppressor was incredibly effective and even when shooting under a shed roof which noticably reflects sound back the supersonic crack was quite mild from behind the shooter’s position.

In the pit, I was standing outside of the cone until it passed.

I agree. The only part of the “cone” that matters is the boundary, the actual shock wave itself. When the shock wave passes, the pressure discontinuity that the shock represents will be interpretted as the sound we hear. If you’re behind the source of the shock, within the cone, you can’t tell that the shock exists. Of course, the non-shockwave sound you hear from supersonic bullet is likely to be louder than a subsonic bullet simply because the energy required to make a bullet go supersonic is going to create a louder bang (all other things the same, yada yada yada…).

For the OP, could the sound you’re hearing be mechanical noise from the weapon? Even a very suppressed weapon with subsonic loads can produce a lot of clatter from the movement of the action.

The rifle I was listening to was bolt action so there are no moving parts save for the striker and firing pin on firing. Even with semi-autos where the action sounds can be heard clearly when using a suppressor supersonic ammunition will sound different than subsonic.

According to Do-It-Yourself Submachine Gun one should use sub-sonic bullets in silencers because they are easier on the muffler. However, the Insultingly Stupid Movie Physics page makes the case that a silenced firearm using supersonic rounds will cause a noticable report no matter how effectively the muzzle blast is muffled, because the bullet will create it’s own sonic boom.

Bonus: if you are ever watching a movie where a character mentions subsonic bullets, tell whomever you’re watching with that subsonics are for use with silencers. Their expressions will be priceless.

Note that while the book available on the amazon link is perfectly legal, it states that it is for ‘acedemic study only’. (Note to mods: I looked, and didn’t see any plans on the page.) It would be illegal to actually make a machine gun.

I don’t think that Insultingly Stupid Movie Physics has done much field research in this area. It may be conventional wisdom that sound suppressors don’t work with supersonic ammunition or with revolvers but this is untrue. Yes, they are most effective with non-vented barrels and subsonic ammo but muffling the muzzle blast alone is dramatically effective.

Movies often get the sound wrong but it’s the type of sound not the low volume that is the problem. Most movies have a sound I can best describe as “thwip.” In Goodfellas where Tommy (Joe Pesci) assasinates Stax (Samuel Jackson) the sound is pretty accurate. A properly suppressed semiauto .22 has only the sound of the mechanical parts moving and virtually none from the shot which probably wouldn’t seem realistic to most people who haven’t heard one.

Very small, maybe even a tiny hijack…

Were the sound effects of bullets accurate in Band of Brothers?

I was profoundly impressed and moved by that drama. Though I’ve never been in combat or in a situation where bullets are whizzing past in every direction, it seemed to me that those sounds were quite real.

Maybe it’s just wishful thinking.

Some one on this board once posted a link to a video which extolled the virtue of a suppressor on a rifle (silencer). The video showed shots taken by the same rifle, suppressed in each case, but with some of the shots using sub-sonic rounds, and the others using high speed rounds. The sub-sonic rounds, with the suppressor, were virtually silent – you heard the bolt action working, a slight metallic click, a brief soft puff, and the clank of the bullet hitting the (iron) target.
The high-speed rounds (the only difference being the “sonic-boom”) gave the same click, puff and clank, but in between was the loud, echoing craaaaakkkkk that is so characteristic of a rifle shot. THAT was the sound of the sonic boom.

(I wish I could find the link - it is pretty cool.)

No, when wearing full ear protection (plugs and muffs) I can hear the action quite clearly. I haven’t had the chance to fire subsonics yet, but they’ll be here by Friday.
The sound I hear is exactly like the sound I’d expect to hear from a bullet striking a cardboard-backed paper target. But I intentionally missed the target and heard the same sound. As a matter of fact, I seem to remember seeing the impact close to the time I heard the sound. I’ll pay more attention this weekend while at the range. I also intend to compare the rifle and my pistol.
Research is such fun! :smiley: