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BULLET BOARDFebruary 2020 G&A 37Understanding suppressed firearms and ammunition.GOING SUBSONICDAVE EMARYSUBSONIC AMMUNITION is not a new thing, but the riseof the .300 Blackout and firearms chambered for it havebrought subsonic ammunition, firearms and suppressorsinto the limelight. The military has been using subsonicammunition for some time to provide stealth in engagementsand to improveclose quarters battle (CQB)communication.There has been purpose-builtsubsonicammunition in existencefor a long time, but onlyin recent years has it beena design criterion for themasses. Many of the oldblackpowder pistol cartridgesand the original40-grain loading of the .22Long were subsonic. Ofcourse, the original 230-grain loading of the .45 ACP was subsonic. There wereguns specifically designed and fielded by the U.S. Officeof Strategic Services (OSS) and British Special Air Service(SAS) for special operations in World War II that used subsonicammunition such as the .22-caliber High StandardHDM; the British Welrod bolt-action pistol in .32 ACP and9mm; and the British De Lisle carbine in .45. The first loadsthat I am aware intentionally developed to be subsonicfor use in suppressed firearms was the 9mm 147-grainfull metal jacket (FMJ) loading for use in suppressed andmodified Smith & Wesson Model 39s carried by U.S. NavySEAL teams starting in 1966 and early HK MP5s elsewhereentering service that same year. The MP5 chambered aload operating at 985 feet per second (fps), while the Mark144 round was specially developed for the Model 39 andfeatured a 158-grain FMJ bullet traveling at 965 fps. Interestingly,the modified Model 39 was classified as the Mark22 Mod. 0 and nicknamed “Hush Puppy.” The Model 39was replaced in 1996 by the the HK Mark 23 in .45. TheMark 144 load was recently recreated and is availablecommercially from Super Vel under the Hush Puppy name.($39, supervelammunition.com)HORNADY SUB-X, .300 BLACKOUT 190-GR. XTP, 20 CT.: $23What is Subsonic? The term “subsonic” refers to somethingthat is travelling slower than the speed of sound(SOS). SOS in a standard atmosphere at sea level is 1,116fps. Above this velocity an object is travelling supersonic.At supersonic velocities the air can no longer get out ofthe way and flow around the object travelling through it.Essentially, air piles up in front of a supersonic object andis compressed to the pointof producing a shock wave,an abrupt pressure anddensity change of the air.If you have ever shot highpower matches or done anypit duty, the loud crack youhear when someone shootsis the shock wave from thenose of a supersonic bullet.The SOS is stronglydependent on temperature.At 80 degrees Fahrenheit(°F), the SOS is 1,138 fps.At 50°F, SOS is 1,106 fps.At 20°F, SOS is 1,073 fps. As you can see, the speed ofsound varies significantly depending on temperature. Thisvariation has to be considered by an ammunition manufacturerwhen designing subsonic ammunition. Not only doesvariation in temperature have to be considered, but alsothe variation in the muzzle velocity of their ammunitionwhen deciding what velocity specification to load to.Subsonic Ammunition Limitations Most of the requirementsfor subsonic ammunition originate with the military.The military has been pursuing subsonic ammunition thatwill function normally in all of their standard-issue firearms,including automatic platforms, for some time. I did a considerableamount of work in the late 1990s and 2000s tryingto make reliable functioning subsonic ammunition forthe AR-15/M4/M16, M14 and SR-25 platforms. We neversucceeded in getting anything in the 5.56mm or 7.62mmcalibers that reliably functioned in these platforms. Wewent as far as 300-grain bullets with a jacketed tungstencore in the 7.62 in an effort to get lower muzzle velocities,but we could never truly get reliable functioning from theM14 or SR-25.The problem lies in the low charge weights that arerequired to produce subsonic velocities. These usuallyresult in low loading density, erratic performance and
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