Subsonic .45 acp help


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rzrrdr
March 28, 2012, 08:41 PM
What's up guys! New to the forum, and have a question for some of the experts here!
Got to thinking about this today, and would like for someone to help me to understand about it.
The Hornady TAP FPD 200gr 45 auto+p ammo is rated at 1055 muzzle fps. According to all info that I have read and heard, the speed of sound is around 1,100 fps. If the ammo is traveling less than the speed of sound where does all the noise come from??? I am saying this and am confused by this, because of the new M&P15 that I have had my good eye on chambered in the .300 Whisper! The concept of the .300 Whisper round is to travel below the speed of sound and have yet to see one in person, but the videos that I have seen online sure have me puzzled!
So I guess, in a nutshell, my question is how can the .300 whisper travel subsonic and be so quiet, but the .45acp travels subsonic also and is soo much louder??
Sorry if I sound stupid, just trying to understand!

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Blackstone
March 28, 2012, 08:52 PM
I believe the sound of a gun comes from both the rapidly expanding gas caused by the explosion, and from the bullet breaking the sound barrier. Sound moderators/suppressors only deal with the former, and can't do anything about the ballistic crack.

As for the .300 whisper, are you sure the rifle doesn't have a suppressor (added on or integral)? I can't think of any other reason why there wouldn't be any sound.
In this video: http://www.youtube.com/watch?v=Zgmy7DxG5h8 the barrel looks heavily modified, probably is why it's so quiet.

BCRider
March 28, 2012, 08:56 PM
It's simply the rapidly expanding POP from the barrel gasses. You forgot about those.

Wahoo95
March 28, 2012, 08:57 PM
AngryHan hit the nail in the head.

Sent from my DROIDX using Tapatalk

The Lone Haranguer
March 28, 2012, 09:04 PM
So I guess, in a nutshell, my question is how can the .300 whisper travel subsonic and be so quiet, but the .45acp travels subsonic also and is soo much louder??

If you fire that .300 Whisper through an unsuppressed rifle, even though it is subsonic, it isn't quiet, it will still make a loud report. It is only quieter by comparison, than, say, a .308 Winchester (which is supersonic) when used with a suppressor, because there is no sonic boom.

Damon555
March 28, 2012, 09:31 PM
If you fire that .300 Whisper through an unsuppressed rifle, even though it is subsonic, it isn't quiet, it will still make a loud report. It is only quieter by comparison, than, say, a .308 Winchester (which is supersonic) when used with a suppressor, because there is no sonic boom.
Yup what he said....

and smokeless powder does not explode, it burns very quickly.

rzrrdr
March 28, 2012, 09:35 PM
Thanks guys! Still dreaming about one of the m&ps chambered in this round. The only drawback would be ammo! Couldn't run down to the local gun shop and pick that up!

The Lone Haranguer
March 29, 2012, 12:33 AM
Another point is that suppressed firearms, even for subsonic cartridges, don't make a barely audible "piff" sound like in the movies. The suppressor more attenuates and diffuses the sound, rather than truly "silencing" it.

asia331
March 29, 2012, 01:27 AM
Speed of sound at 59F/15C is 1116fps

Sam1911
March 29, 2012, 08:36 AM
As others said, you have both the POP of the rapidly expanding combustion gasses escaping from the barrel suddenly (Factor #1), AND the supersonic crack of the bullet if it is going fast enough (Factor #2).

The silencer only spreads out and slows the escape of those gasses (turning a lot of the energy into heat, by the way) and so slightly reduces that "Factor #1." A good silencer can reduce the first part of the sound signature from the blast of a gunshot to more like a door slamming hard. Not "silent" by any means.

(Though with lightly loaded .22 rimfires, they can get VERY quiet indeed.)

"Factor #2," the supersonic crack happens outside the gun and a silencer can't help that. If the bullet is going over the speed of sound it will happen, and it can be painfully loud.

Ironically, a silencer can HURT, here. Adding a can creates an effect often called "suppressor boost" which means the silencer acts more-or-less like a few extra inches of barrel, and that will bump up the bullet's velocity. If your ammo is going very close to the speed of sound anyway, a silencer can add enough velocity to push it over!

jmorris
March 29, 2012, 10:02 AM
(Though with lightly loaded .22 rimfires, they can get VERY quiet indeed.)

Yep, I have a 22 bolt gun that makes more noise dry firing than it does shooting CB longs (firing pin on brass vs steel and zero report)

BCRider
March 29, 2012, 03:43 PM
One thing to think about with a rifle that is shooting subsonic projectiles. With that low a muzzle velocity the gun is going to become a very short range sort of rifle. The simple act of gravity on the bullet in flight will make the trajectory of anything much over a 100 yard shot rather "rainbow" like. A 200 yard shot will need a serious amount of holdover or sight adjustment.

Or am I missing something when it comes to the 300 Whisper round? I see that they come in loadings which have lighter supersonic bullets as well as heavier subsonic options.

A while back I was shooting my 1911 at a 200 yard gong. With the gun sighted for close in shooting I found I had to use a crazy amount of holdover to range the shots in for the 200 yard gong. Something like 6 feet worth above the gong.

sawdeanz
March 30, 2012, 02:53 PM
To sum it up, if you want the lowest report, you need a suppressor to minimize one source of sound, and sub sonic ammo to avoid the sonic boom.
Neat thing about .300 and .45 is that you can lower velocity to below the sound barrier just by using bigger bullets, so you don't lose too much power (small and fast vs big and slow).
But yes I imagine that the holdover will be much greater, I think there is a reason the AK is considered a 300 yd gun and the AR is considered and 800 yd gun. The .300 is more like AK from what I hear. (though many will debate whether that is really much of a disadvantage because what living thing will you be shooting at from beyond 300 yds?)

ku4hx
March 30, 2012, 03:46 PM
The crack is the same as thunder: rapidly expanding gases. In the case of thunder the lightening heats the air and its rapid expansion causes the thunder clap. Same with a gun: rapidly expanding gases created as the powder burns and escapes the barrel.

"Silencers", better called "suppressors" calm the rapidly expanding gases thereby lowering the decibel output. Usually this is accomplished by what is known as a "torturous path" configuration that basically "rubs off" the energy of the expanding gasses.

If all the sound came from breaking the sound barrier, an MV of less than the speed of sound (at whatever elevation you happened to be) would effectively produce no report upon firing. In fact, the bullet breaking the sound barrier is a relatively low intensity sound. Bullwhips crack because the tip breaks the sound barrier, and that sound is not enough to required ear protection.

The Lone Haranguer
March 30, 2012, 08:55 PM
And no, suppressors don't work on revolvers. (Except maybe the Nagant.)

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