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. Once the transonic region is surpassed (above 400 m/s) the sound level rise flattens out considerably. 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. 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. These may look like small increases in sound level, but dB is a logarithmic scale. 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. The graph shows it all.Īs you can see, above 300 m/s (about 984 ft/s) there is a sudden and very steep rise in the level of sound. Muzzle blast is not included since the gun is well suppressed. In other words, sonic and subsonic bullet sound level is not an all or nothing thing. Small variations in velocity UNDER mach 1 can have a BIG impact on the sound of the bullet. This is even true with all else being equal (gun, powder, suppressor, bullet). Not all bullets below mach 1 are the same level of quiet. One thing in particular it contained was very interesting, and confirmed something I had noticed with subsonic loading. 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. The key to this test is the sound level was set manually, so the actual sound level between each velocity can be heard. There are more details on the test and the equipment in the video description. 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.
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