Supersonic vs trans-sonic vs subsonic

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Supersonic vs trans-sonic vs subsonic

Post by Deude_Mann » Sun Jun 16, 2019 12:51 pm

This is a Youtube video of a test I conducted of the different sound levels with 220 gr 300 BLK loads shot through a suppressor with an AR15. There are more details on the test and the equipment in the video description. The key to this test is the sound level was set manually, so the actual sound level between each velocity can be heard.

So while researching subsonic loads in preparation for my next set of 300 BLK handloading, I stumbled upon an interesting web site that included a lengthy, but informative discussion on suppressors in high power rifles, including a discussion on subsonic ammunition. One thing in particular it contained was very interesting, and confirmed something I had noticed with subsonic loading.

Not all bullets below mach 1 are the same level of quiet. This is even true with all else being equal (gun, powder, suppressor, bullet). Small variations in velocity UNDER mach 1 can have a BIG impact on the sound of the bullet. In other words, sonic and subsonic bullet sound level is not an all or nothing thing. Muzzle blast is not included since the gun is well suppressed. The graph shows it all...


As you can see, above 300 m/s (about 984 ft/s) there is a sudden and very steep rise in the level of sound. In fact, at 1050 fps, the sound level has risen from 90 dB to 95 dB, and at 1100 fps it has risen to around 103 dB. These may look like small increases in sound level, but dB is a logarithmic scale. So, for example, the difference between 90 dB and 103 dB represents over double the PERCEIVED sound level; the bullet sound seems twice as loud. This trans-sonic region just below and just above mach 1 (300m/s to 400 m/s) has very large increases in bullet sound as a function of velocity. Once the transonic region is surpassed (above 400 m/s) the sound level rise flattens out considerably.

So, when loading subsonic ammunition, if you want it to be really quiet, and are not trying to capture every little ft*lb of energy (like for plinking/target ammo) aiming for a load at or below 980 fps or so is a good idea. I have noticed big differences in sound level between loads at 950 fps and 1050 fps for example, even though both are 'subsonic'. The difference is the slower bullet is quieter than the AR's action, and the faster one is louder than the AR's action.

This graph is originally credited to Alan C. Paulson in his book Silencer History and Performance, Vol. 1. ... 0873649095

It was displayed on this web page here. This web site is a long read, but there is a lot of interesting and useful information contained within. ... rifles.htm

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Re: Supersonic vs trans-sonic vs subsonic

Post by dellet » Sun Jun 16, 2019 3:38 pm

Bullet shape makes a huge difference, just a canalure, pressure or lube groove can cause the air moving over the bullet to reach super sonic speeds.

Part of what you noticed is that bullets under 980fps are truly subsonic, where at 1050 they are transonic. Transonic flight causes a lot of turbulence/noise and is really the absolute worst velocity to shoot in from and accuracy and noise standpoint.

It’s good to see some of that documented and recorded.
300 Blackout, not just for sub-sonics.

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Re: Supersonic vs trans-sonic vs subsonic

Post by alamo5000 » Sun Jun 16, 2019 11:54 pm

Great post.

I love nerding out on stuff like this.

I am not sure if you watch the YouTube channel "Smarter Every Day" but they did some videos a while back where they actually captured images of this.

It's absolutely fascinating stuff.

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