Sub-Sonic Ammo

GSR+ Blog 2

Sub-Sonic Ammo

What is sub-sonic ammo and why is it needed?

We will need to look at a few things to understand the practical application.

First, anything accelerating to over the speed of sound will create a sonic boom.  This includes rockets, jet plans, the occasional car and of more interest to us, bullets.

     According to our friends at Wikipedia and my high school physics book.  (yes, I’m one of those geeks who kept his physics textbook.) 

     At 32° F the speed of sound is 1,086 ft/s or 740 mph in dry air at sea level which is 14.7 pounds per square inch (psi).  Since dry air (zero humidity) is hard to find on land or sea, we need to assume that the average humidity will be closer to 50%.  Also, if we apply the rule that sound travels faster the denser the medium, we have to do some estimating.  Another thing to consider is that at 8000 feet elevation air is thinner by half, or only 7.35 psi. so, when you’re in the mountains sound will travel slower.  

     Did you ever wonder why sound seems to carry more at night?  Me too.  Cooler air is more dense so,  the sound travels faster.  Also, things seem louder at night because there is less ambient or background noise to interfere.  It’s sort of like using white noise to lessen the ability to hear other random noises to aid in sleeping.

     Thanks to our friends at ScienceFacts.com for this great diagram. 

     Look at the compression points.  A bullet exiting a barrel will be causing a compression

point as it, the air in front of it in the barrel, and the expanding gas behind it hit the surrounding air.  But the bullet will pass through the compression point because the air can’t compress beyond a certain speed.  That is, it has an absolute maximum compression for sound.  This action causes the sonic boom when the air tries to fill the vacuum left behind the compression point by the moving bullet.

     Now, how do we as responsible shooters know the velocity of our ammo?  That’s right, by checking the manufacturer’s data on the box, checking the reloading date if we reload, or by using a ballistic chronograph. 

     The baseline number for the speed of sound, is less than 1,100 feet per second (ft/s).  So sub-sonic bullets will not cause a sonic boom.  This translates to quieter ammo as it travels through the air. 

     When is this important?  If you’re using a suppressor with handgun ammo and wish to avoid the sonic boom.  Or if you don’t want to bother your neighbors as much when target practicing in the back yard.  You might also be thinking that if you are hunting varmints, it could be helpful. Based on the sensitive hearing that most animals have not making a sonic boom probably won’t help.  Getting the bullet there way before the sound gets to the varmint is probably your best strategy. You can make that determination for your personal needs.  Slower does mean quieter, or more accurately, “Less Loud.”  Be careful not to under pressure your reloads or you can cause misfeeds, failures to eject, or instantaneous ignition of powder.

     There is no way to get around it with rifle ammo.  For example: if you’re shooting 7.62 NATO rifle ammo (.308 or 7.62x51mm) the average speed of most cartridges is going to be at least 2300 ft/s. 

     If you’re using .22 Long Rifle (lr) in a handgun or rifle the velocity will be just over the 1,086 ft/s speed of sound for standard ammunition.  Always remember that a longer barrel allows for more expansion of the burning gun powder so higher velocity.  5.7x28mm generally has a muzzle velocity of 1,450 and 1,760 ft/s, well over the speed of sound. 

     Let’s go to the other end of handgun ammo velocities.  25 ACP or 25 Auto comes in somewhere between 540 and 900 ft/s.  380 ACP is well under our sub-sonic threshold.  Some 9mm Luger is below and some is above.  38 Super is all over by at least 90 ft/s.  What about 38 Special?  The majority of loads are Sub-sonic, as are some .357 Magnum loads but most 357 loads are well over 1,100ft/s.  40 Smith & Wesson is generally near sonic so, check the packaging.  10mm is on the line so, again check.  44 Remington Magnum will tend to be faster.  All .45 ACP is well below.  Most 45 Colt is also below the speed of sound but not 454 Casull.   (Velocities are generalized from Lyman Reloading Handbook 49^th Edition.)  

     Don’t pay extra for “Sub-Sonic” ammo when the caliber you need is already slower than the speed of sound.

I would love to hear your thoughts.  Please email me at GSRplusTraining@gmail.com Put “Brian’s Blog Sub-Sonic” in the subject line.