On-Target Performance of Subsonic Ammo in Suppressed Rifles – Hype Versus Reality

As a follow-on to another article about the causes of gunshot noise that I posted in my blog, I’m posting this article about the hype versus reality of terminal performance of subsonic ammunition fired from suppressed rifles. Many people, including those who are experienced in the field of physical security, believe in some sort of magic surrounding suppressed weapons and their effect on targets when the targets are hit by projectiles (bullets) fired from those weapons. In this article, I show why suppressing a rifle and shooting subsonic ammunition from it usually produces worse performance (kinetic energy delivered on the target) than a rifle with standard rifle ammunition.

Protecting against rifle fire

In a past consulting job, my customer asked me to test many different ballistic protection schemes against penetration by projectiles fired from three different rifle systems he considered the primary threats. Possible attacks on his business’s protected spaces by homegrown terrorists or disgruntled customers (!) concerned him. After I performed the tests, I provided him with the following table of information regarding the ammunition I used:

A listing of the test results from testing I performed to determine bullet kinetic energy

I handloaded the cartridges, weighed each projectile and selected only those within one-tenth grain of the specified weight. I chronographed each shot, and the values in the table are the average velocities from the cartridges in the test. The kinetic energies I calculated from the formula E_k = ½ mV².

Does a suppressed rifle shooting subsonic ammunition perform better against a target?

After looking at the penetration results and the tabular information about the cartridges, my customer asked me, “Do you think ammo fired through silenced weapons would perform better?” My immediate reply was “No way” and I briefly explained why I thought so. He was an intelligent person, and because of his position in his company and his life experience in law enforcement he had a better than average knowledge of firearms. So, while I was explaining to him that from a kinetic energy standpoint I didn’t think cartridges fired through suppressed weapons would outperform “standard” rifle fire, I was thinking that I needed to thoroughly investigate the possibilities through some research and calculations (and, possibly, additional testing).

My explanation did not satisfy him, and he said that suppressed fire concerned him, since part of his company’s defense in depth concept used a shot location system to inform his first responders in case of an incident. At that time, his location system wouldn’t reliably spot the origin of suppressed gunshots.

Hollywood hype

In addition (and here’s the rub), TV shows and movies have “Ninjas” dressed in black running around shooting suppressed weapons that are highly effective against everything (and everyone) they shoot. So, what were the facts regarding the performance of subsonic ammunition fired through suppressed rifles?

I agreed to do some research and get the information back to him so that he could decide on further testing. So, I began with my customer’s question, “Can ‘silenced’ rifles be nearly silent and perform ballistically on a par with (or better than) rifles without suppressors?”

First, I ruled-out supersonic ammo because the sonic “snap” would activate the shot location system. Admittedly, the sound might confuse the system as to the shooter’s location, but my customer agreed, nevertheless, that the sound would cause an alert.

Next, I looked at a range of commercially available subsonic ammunition and ammo where handloading produced projectiles at subsonic muzzle velocities. For each of these ammunition types, I chose a cartridge/projectile combination likely to produce and maintain higher E_k better than the others. Another consideration was whether the combination was easily available for purchase, realizing the two categories might overlap. As a result, I considered the following ammo, loaded to be subsonic: .38 Special (carbine), .308 Win/7.62 x 51, .45 ACP (carbine), .458 SOCOM, .300 AAC Blackout, 7.62 x 39, 5.56 x 45.

Analysis

I used the following considerations/assumptions as I began my analysis:

.38 Special is a handgun round, and even through a carbine, only the lighter bullets made for the cartridge loading would produce supersonic performance. Therefore, there are several possible selections for test cartridges.
.45 ACP is a handgun round, and as with the .38 Special, only the lighter bullets will produce supersonic performance.
.458 SOCOM is a rifle round designed to fit into magazines conforming to the AR-15 magazine form factor. Rifles and AR-15 upper receivers designed for the round are available from a couple of companies, as is the ammo. The projectile weights vary from 300 to 600 grains, with only the 600 grain ammo advertised as subsonic.
.300 AAC Blackout. Similar in effectiveness when using supersonic ammo as the 7.62 x 39, the same twist rate works well for supersonic as well as subsonic ammo with bullet weights of 220 grains and below. Heavier bullets require faster twist barrels.
7.62 suppressed (7.62 x 39 mm, the rifle round created for the AK-47, but normally supersonic). The key to making this cartridge work is to have the right combination of fast-burning propellant, bullet type, bullet weight and rifling twist rate in the rifle to achieve subsonic velocity with stable projectile flight and safe powder burn in the cartridge. With some experimentation, one can create a subsonic 7.62 x 39 round for a rifle that has the needed barrel twist rate.
5.56 suppressed (5.56 x 45 mm rifle round, normally supersonic). Same considerations as for the 7.62 suppressed, above.
.308 suppressed (.308 Winchester or 7.62 x 51 mm rifle round, normally supersonic). Likely will require bullet weights greater than 190 grains; same considerations as for the 7.62 suppressed, above.

Reality

As a first cut to weed-out some of the alternatives, I assumed that hard-surface penetration would require full metal jacket projectiles. Next, in order to make some logical comparisons to the rifle rounds I already tested, I then calculated the muzzle energies of several bullet and cartridge combinations using information gathered from ballistics software and loading information from reputable sources.

Ranking of subsonic cartridges based on muzzle kinetic energy

Kinetic energy ranking of bullets from subsonic cartridges

Note that many of these ammunition combinations are not commercially available and must be handloaded. The .458, .308, and 170-grain .38 ammo combinations may be particularly difficult, even for a handloader who has a weapon with the right barrel twist rate and who is willing to take the experimentation time to achieve subsonic velocities.

Based on this information, I recommended to my customer that if he wanted a test series using subsonic ammunition, it would be best to use a rifle firing the .300 AAC Blackout cartridge. Of the subsonic ammo combinations I examined, It would be the most easily obtainable for an attacker.

Comparison with the test data from our previous tests using supersonic cartridges, however, led me to believe that this additional test was not required from an engineering evaluation standpoint. Using kinetic energy and bullet shape as the deciding factors, the 7.62 x 51 NATO M80 Ball test cartridges produced muzzle kinetic energies more than twice the largest energies produced by the subsonic cartridges in the table, above. The 7.62 x 39 Russian and the 5.56 x 45 NATO M855 cartridges previously tested produced muzzle kinetic energies larger than the subsonic .458 SOCOM cited, above. Moreover, contrary to the .458 bullet, the two NATO cartridges are recognized for having at least some penetration capability against steel targets.

The bottom line

My customer agreed that no further testing was required. Once again, Hollywood mischaracterized firearms. The only advantage subsonic rifle ammunition offers when compared to standard, supersonic ammo is noise reduction when fired through a suppressor. Even the muzzle kinetic energy of the biggest subsonic “thumper” (the .458 SOCOM) I examined falls short of the tiny 5.56 x 45 mm military ball. When fired at hard targets, the .458’s much larger diameter is of no help, either, as it merely spreads-out the penetration load on the target’s surface.