Shoot Without Noise

7.62x39 "Thumper" Project Rifle

 

The road less traveled sometimes is the more interesting journey.
 

You are about to enter a region of the reloading world where no official published data exists, where truths, half-truths and old shooter's tales are inexorably entwined. The entire miscellany of legend and fact is bound up in the threads of history, which must be carefully unwrapped in order to get at the truth. There is a general fascination in the firearms world with the unusual and the exotic. This tale involves both.

Ever since firearms became practical tools, methods have been explored to minimize or eliminate the noise associated with muzzle blast. While it is not possible to completely eliminate (silence) all the various noises involved in the firing process, it is possible to greatly reduce the magnitude of the muzzle signature.

The earliest methods of noise reduction were developed shortly after the introduction of the metallic cartridge with major innovations in quiet shooting being established during the period encompassing WWI and WWII. One sound damping method is widely known and another has been relegated to the sidelines of history as a firearm oddity. It is the second, less known, method that will be investigated here.

 

The well-known method of reducing muzzle signature is, of course, the Silencer or more correctly the Suppressor, invented by Hiram Maxim (right) just prior to WWI. However suppressors did not see widespread use until their adoption by various Special Forces during WWII.

Silent shooting method number two was pioneered by the Russians during WWII and then afterwards kept alive and further developed by the Finns. Ironically, it has been Finland, with no restrictions on the ownership of Suppressors that has done the most to further the development of subsonic and silent shooting technology.

The basic theory of subsonic technology is the use of small charges of fast burning pistol powder to propel heavy projectiles at velocities below the speed of sound from otherwise conventional rifles. The small powder charge is entirely consumed within the bore resulting in an extremely reduced muzzle report while the large, heavy projectile yields an effective range measured in hundreds of yards.

Why shoot subsonic? A quiet rifle is less intimidating to a novice making marksmanship instruction easier and friendlier. Quiet shooting can improve relations with the neighbors and reduce the damage to hunters' ears. Reduced muzzle signatures are viewed as being less dangerous and more humane for tactical, law enforcement and conservation purposes.

 
  PROJECT RIFLE

Maxim Silencer
  Unable to find much in the way of factual or recent information through open sources, I decided to build a rifle specifically to explore the world of subsonic shooting. It started with an unused Mauser 98K action. The project was specifically limited to readily available, off-the-shelf components. This simple constraint would drastically reduce costs by allowing the use of an existing cartridge case, a commonly available barrel and off-the-shelf dies and bullets, making the idea accessible to anyone. Once the basic constraints for the project had evolved a calibre and cartridge had to be decided on.
 

Original Mauser 98K project rifle (top) and its replacement, Mini Mauser
  RESEARCH    
 

Perusing firearms history books I discovered the .45 caliber WWII DeLisle Carbine. The inherently subsonic .45 ACP throws a big heavy bullet but it has the aerodynamic shape of a brick. Modern versions include the integrally suppressed Destroyer Carbine (9mm) and Ruger 77/44 (.44 Magnum), which also use large bus-shaped bullets. I was looking for something a little more aerodynamic to stretch the rifle's effective range beyond 200 yards.

Limiting velocity to less than 1000 fps meant that bullet weight had to be substantial to carry hitting and penetration power out to longer ranges. The .50 cal and .338 cal fit these parameters but bullets, barrels and brass become a costly endeavor. Going this route would also mean a custom wildcat cartridge, which was not within the project parameters.

Subsonic loading techniques actually began in WWII with the Russian Moisin Nagant rifle's 7.62x54R cartridge. There are more different .30 cal cartridges than any other caliber giving the best chance of finding a suitable case. The .30 cal also has the benefit of having large number of different bullet weights and styles to choose from and barrels are easy to get.

With the caliber chosen a cartridge case and some reference load data was still needed. There is a fair amount of basic information on subsonic loads to be found on the Internet, however, in our litigious world, there is little published load data for specific cartridges. Where data was published, it was almost always for use with lightweight bullets or cartridges much larger than I wanted to work with. It seems very few people have experimented with heavy weight, subsonic bullets fired from medium sized cartridges.

Trolling through load manuals and cartridge specifications I made a startling discovery. The 7.62x39 case (case image) volume is almost identical to that of the .44 Magnum with a bullet seated. Additionally .44 Magnum load data included bullet weights from 180 grains to 220 grains, within the range I wanted to explore. Finally here was a suitable .30 calibre cartridge and load data that could be adapted to it. Note: Only very experienced handloaders should attempt to develop new load data.

Side Note: Other .30 caliber cartridges worth investigation for suitability as subsonic platforms include .30 Herret (case image), .30 Kurz, and 308x1.5" (case image).

To stay within the project's "off the shelf" limitation, a factory 1:10 twist .30-06 take-off barrel was chosen. This twist rate will stabilize a 220 grain round nose bullet at 950 feet per second. A minimum length 18 inch barrel was specified to give the rifle lightweight and fast handling characteristics. It also proves the capacity of the rifle to be easily adapted to an integral suppressor without excessive over-all-length and it is the shortest barrel length allowed by Canadian law.

The benefit of the 7.62x39 case is that it can be easily adapted to a number of platforms and will function through a semi-automatic rifle like the AR15/M16. The 7.62x39 uses a .311 calibre but it can be easily adapted to the slightly smaller but much more common .308 calibre. The .308 bore allows a wide variety of bullet weights and styles to perform various jobs. .311 sized bullets can be shot through a .308 bore without a problem. In these respects the subsonic 7.62x39 (.308) is much more versatile than others.

Exotic rifle systems made primarily of Unobtanium are useless to all but the largest and best-funded law enforcement agencies. The use of exotic cartridges also means that policing up brass becomes critical to avoid revealing the user's identity. The 7.62x39, being in use with more than half the world's armies and a huge majority of the world's terrorists and freedom fighters, makes empty brass an untraceable entity.

  SUBSONIC AMMUNITION

 

  Subsonic 7.62x39

Boat-tail bullet seated backward for better terminal effects.
 

When I started this project four years ago, you couldn't just pop down to the local gunstore and buy some subsonic ammunition. Things have changed a bit in the past few years with a number of commercial subsonic loadings available in military calibers.

Even with its current popularity and availability, there is a lot of work to be accomplished in the world of subsonic ammunition. Most commercial subsonics use off the shelf, conventional bullets that are limited by the twist rates that will stabilize them and the amount of terminal performance they can deliver in soft targets. There is little if any subsonic rifle reloading data available in manuals or on the internet.

There is still a role for experimentation and handloaded ammunition before the big companies take over and every hardware store from here to there has subsonics on the shelf.

Loading subsonic ammunition is not nearly as simple as metering a few grains of pistol powder into the case and seating a big, heavy bullet. Developing new load data where none exists can be a bit nerve wracking. Potential dangers are minimized through specific techniques involving case, bullet and bore preparation as well as powder choice for burning speed and load density. Go here for a more detailed discussion of Subsonic Loading techniques.

Initial subsonic test loads involved several different pistol powders pushing 200 grain bullets to find safe, benchmark data. Loads were started high and worked down a half grain at a time with load density and muzzle velocity monitored for each round. Ultimately it was possible to safely reduce muzzle velocity to 875 fps. Armed with this data, loads were increased to 950 - 1000 feet per second with confidence that the working ammunition was safe.

  Velocity should be 50-100 fps less than the speed of sound or about 950 fps. This puts velocity below the transonic range where the supersonic crack begins to be generated. The sonic crack is not generated at exactly the speed of sound. The sound increases in volume across the transonic range starting about 92-93 percent of the speed of sound based upon research by Al Paulson. Also keep in mind that a sound suppressor (silencer) may create about 30 fps of freebore boost, which in turn argues for lowering projectile velocity an additional 30-50 fps. At about 50 fps above the speed of sound the sonic crack reaches maximum volume.
  SUBSONIC BULLET CHOICE    
 

Subsonic loads are a constant battle between opposing forces. With velocity fixed below 1000 fps, terminal energy can only be increased by increasing bullet weight. Increased weight eventually runs smack dab into bullet stability problems. The longer and heavier the bullet, the more difficult it is to stabilize. We need to walk the line between bullet weight and stability.

Bullet weight can be increased without increasing length by changing the shape from pointed to round nosed or even a full wadcutter design. However each of these changes has it's own limitations. Wadcutter shapes, with their square nose, cause feeding problems in rifle actions and round nosed bullets do not perform terminally as well as we would like them to.

In flesh, a sharply pointed subsonic bullet will simply push tissue aside without causing much terminal damage. A blunt bullet nose with a sharp edge causes more terminal damage because it cuts tissue as it passes, leaving the maximum permanent cavity and wounding potential. Jacketed hollow point bullets are designed to operate within a narrow velocity range and most will not expand reliably at low velocity.

From observation, impact energy doesn't appear to be as important to a subsonic bullet as it is for conventional velocity bullets. The subsonic bullet relies on its shape to cut a wound path. Lighter weight subsonic bullets have more than enough energy to traverse a tactical target due to the lack of impact deformation. Lighter bullets offer a flatter trajectory, which is quite important.

The ideal bullet design and weight depends on the operating parameters for the rifle but generally it can be advantageous to sacrifice some weight and energy to gain stability and trajectory. In a .30 calibre system, a 168 grain BTHP flying backwards is probably the best choice. This round will feed reliably in a bolt action and give the best terminal performance while allowing accurate shots to 200 yards.

Coincidentally the BTHP flying backwards is the shape that is the quietest aerodynamically. Studies have shown that a teardrop shaped projectile produces the least flight noise. A BTHP bullet travelling backwards is as close to a teardrop shape as can be had in a commercially available bullet. A subsonic projectile makes a distinct flight noise that is easy to track back to the shooter. The only fly in the ointment is that subsonic bullets are most accurate with a flat base so it can be difficult to get backward loaded HPBT's to shoot accurately.

Click here for additional information on Subsonic Bullets

  LOAD TESTING

Subsonic Oddities

Shoot the same bullet forwards and backwards and the first thing you'll notice is a blunt nose reduces the ballistic coefficient causing a more pronounced trajectory. Velocity comparisons at the muzzle and downrange demonstrate that subsonic flight produces a high ballistic coefficient (about 0.500), much higher than the identical projectile would have at conventional velocity. This varies somewhat with the weight and shape of the bullet.

One of the really weird phenomena to contemplate, is that a subsonic bullet can be unstable flying forward yet be stable flying backward. This has to do with the relationship between the center of gravity (CG) and the center of friction (CF). In long pointed bullets the CG is often in the rear half of the projectile. If the CG is in front of the CF (as it would be in a bullet flying backwards) the bullet will be stable for the same reasons a badminton bird is stable, the weight at the front drags the lighter tail along behind it.

Another subsonic oddity involves a bullet that is stable at 100 yards but begins to show increasing instability with distance, culminating in sideways impacts at around 200 yards. Conventional theory states that a bullet, which is stable out of the muzzle, will continue to be stable throughout its flight, with stability increasing because forward velocity decreases faster than rotational velocity. Interestingly unstable projectiles maintained reasonable accuracy reasonably with groups ranging from 1.85 MOA to 2.5 MOA. This also flies in the face of conventional wisdom.

Spark photograph of a subsonic bullet (above) shows lack of supersonic shockwave as is visible on the image of the supersonic bullet (below)

 

Initial tests included a number of different bullet weights and shapes ranging from 150 grain FMJ-BT to 220 grain round nose. The two bullets eventually settled on were the 150 grain FMJ-BT and the 180 grain round nose.

The lighter weight boat-tail is stable in flight both forwards and backwards and surplus .30 cal pulled bullets are cheap to experiment with. The heavier round nose bullet is the most stabilized of all bullets and offers more weight and downrange energy. It has an additional benefit of flying an almost identical trajectory to the 150 grain FMJ-BT in the forward configuration.

Experimentation with subsonic projectiles shows there is a large grey area between a stable bullet and an unstable bullet and the parameters governing stability at conventional velocities do not apply. A lot of time was spent testing different bullet sizes and shapes to determine stability. Generally lighter weights and round nose shapes gave the greatest stability both in flight and terminally. Go here for more information on bullet stability.

Investigation into the issue of projectile stability suggests that subsonic projectiles require a much greater degree of stabilization than supersonic projectiles. Ballistic stability calculators show that a stability factor of 1.0 - 1.3 is required for supersonic flight stability. Experimental data shows that subsonic .30 calibre projectiles are not flight stable with a stability factor of less than 2.0. Stability depends greatly on the shape and fore-aft balance of the projectile. A bullet with stability as high as 2.65 was found not to be stable yet a specially designed subsonic bullet with a stability factor of only 1.36 was stable.

Subsonic projectiles with a stability factor of about 2.0 - 2.2 can be stable in flight but display terminal instability. During testing heavy plastic and steel drums were used as target stands. Examination of the drums revealed that marginally stable subsonic bullets usually yawed through 90 degrees within 24 inches after impact, to exit the drum completely sideways. Amazingly enough the sideways bullets traveled in a reasonably straight line for a short distance after exiting the drum.

The heavy plastic construction of the drums represented a difficult test of a subsonic bullet's ability to penetrate. None of the bullets were contained by the drum. Unlike steel, plastic can give and stretch before it breaks. The side of the drum was about 1/8 inch thick with the plastic being of very solid construction. The entry holes clearly bore the rifling marks engraved on the bullet. The exit holes were much more impressive because the bullet had yawed completely sideways to present the maximum possible striking area to the drum side. With no sharp edges to help cut through the hard plastic, the bullet's retained energy simply bullied it through the thick plastic.

At every shot a shower of concrete dust erupted from behind the drum. Not only did the subsonic bullet have the power to penetrate both sides of the drum but it was doing its best to beat up the concrete blocks behind as well. This suggests that subsonic rounds could easily penetrate vehicle bodies and other light cover while retaining the ability to fly straight.

Getting accuracy out of subsonic loads turned out to be a challenge. Accuracy is not comparable to conventional ammunition and it tends to get worse with distance.

The practical limit of subsonic range seems to be about 200 yards. It is certainly possible to shoot further but range and wind estimations become absolutely critical. The artillery like trajectory of means that even a minor range error could result in a miss. For example, from a 100 yard zero, bullet drop at 150 yards was 16" and drop at 190 yards was 39". At 100 yards a 180 grain subsonic bullet is falling almost 6.5 inches every 25 yards. Windage estimation is equally important because the slow moving bullets leave a lot of time for the wind to do its damage.

  FINAL THOUGHTS
 

The 7.62x39 subsonic rifle has proven to be amazingly versatile. It can operate with conventional ammunition, specialty high velocity/low signature ammunition and subsonic ammunition. Bullet styles and weights can range from 110 grain polycarbonate tipped to 220 grain solids to fill just about any mission essential need.

Subsonic 180 grain and 200 grain loads produce unsuppressed sound pressure levels (SPL's) of 152 dB, which is the same as an unsuppressed .22 Rimfire pistol. The huge noise reduction factor (approx 12 dB) over conventional ammunition (roughly 60% less noise) is bound to expand the tactical applications of the subsonic rifle. The addition of a suppressor should bring the muzzle signature down to the arena of a suppressed .22 rimfire pistol (approximately 130 dB). Sound Test Data courtesy of Al Paulson.

Windows and glass present a conundrum to the sniper. The target can be seen clearly and glass offers little protection from even a thrown rock, yet it presents an incredibly hard surface for a supersonic rifle bullet, especially for those designed to expand upon impact. The trend of law enforcement agencies utilizing solid copper bullets for glass penetration has not at all addressed the potential problems of deflection and secondary missiles.

Low velocity projectiles reduce the number and speed of secondary missiles created by the glass impact in addition to reducing impact deformation and deflection. Even if a subsonic bullet is deformed or destabilized by the initial impact, testing has shown that destabilized subsonic bullets continue to fly in a reasonably straight line.

The primary tactical use for a large caliber, subsonic projectile in Law Enforcement would be to engage targets through glass or other "penetrable" cover such as vehicle bodies. Supersonic projectiles can be deflected or even disintegrate as a result of the high velocity impact with glass or sheet metal, missing the intended target completely. This effect is magnified by the use of frangible projectiles and was demonstrated dramatically in one highly publicized police sniper shooting where the hostage taker was missed completely by the bullet and went on to shoot several of the hostages as a result. The news video of this incident was broadcast around the world, the sort of publicity that most police forces would surely like to avoid.

 

7.62x39 "Thumper" during field testing
  Author's Note: During the preparation of this article a very real negative stigma attached to the loading of quiet ammunition by civilians was revealed. Ironically most handloaders have no issue with building bigger and faster cartridges. Which is more lethal; the subsonic rifle with a limited range and non-expanding bullet or the large magnum rifle able to engage targets at a mile with polymer tipped bullets that disintegrate explosively? Subsonic loads are another facet of reloading that do not render a firearm any more or less deadly.