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The Loading and Safe Use of Subsonic Ammunition
"The loading, or using of subsonic ammunition should not be undertaken lightly. Consult your local laws, and be cautious. The incorrect or illegal use of sub-sonic ammunition, or suppressing devices, should be done in a legal, and safe manner. Not doing so may present significant risk of bodily harm or loss of liberty or both." The Owners / Operators of this Blog - neither endorse, nor recommend the use of these products.
The goal of this article is to explain techniques used to safely develop and load subsonic ammunition without risking life and limb.
The ultimate danger in subsonic loading is a phenomenon known as
Secondary Explosive Effect (SEE). SEE can destroy the strongest rifle action and it can happen on the first shot or the tenth. In fact it is more likely to happen after the bore is dirty from the first few shots. SEE is the result of slow or incomplete ignition of small amounts of smokeless powder. The powder smolders and releases explosive gases which, when finally ignited, detonate in a high order explosion. The warning sign for imminent SEE is a hangfire or delayed ignition.
There are a number of techniques that can be used to prevent or delay the onset of SEE. These involve specially prepared cases as well as bullets, in addition to selecting a suitable powder and primer.
Let’s begin with the easy stuff. All reduced velocity loads should start with a magnum primer. The hotter and longer duration flame ensures the small powder charge lying in the bottom of a large rifle case is properly ignited. Testing revealed that accuracy is increased 8% with magnum primers.
Reloading manuals contain minimum powder charges for a reason. Going much below the minimum load with a slow burning rifle powder can trigger SEE. Subsonic loads use fast burning pistol powders, the faster the better. This ensures the powder is easy to ignite and will be totally consumed before the bullet leaves the muzzle. Not only is this safer but it is quieter as well. Quieter is always good when operating in the subsonic realm.
A secondary consideration when choosing a powder is its bulk density. That is the amount of volume a given weight of powder will occupy in the case. Bulk densities are typically measured in grains per cubic inch; with the lower number meaning the powder is more voluminous. Fluffy (low bulk density) powders like Red Dot take up much more room in the case. Load density is safety in a subsonic load. It is suggested that loads never go below 30 percent load density (powder occupies 30 percent of the case volume) unless you want to dance with the devil and the deep blue SEE.
Load density can be measured by dropping a length of wood dowel into the loaded case. Mark the dowel where it meets the case mouth. Then pull the dowel out and compare it against the case. Keep notes on load densities for every powder and case combination so that safe loads can be identified at a glance before they are loaded. Don't go below 30% load density!
Powders suitable for subsonic loads include: N310, N312, Bullseye, Clays, Titewad, HP38 and Red Dot. There are other suitable powders available but these are the best to start out with. Red Dot is probably the best powder to work with because it gives the highest load densities. The Vihtavuori powders (N310 and N312) are particularly interesting because they burn very quickly and cleanly and they exhibit exceptionally stable performance over an extremely wide range of environmental temperatures.
Powder burn rates are important because the low pressures generated by subsonic loads are typically not sufficient to properly seal the case in the chamber. Faster powders create better sealing and less sooty blow-by on the outside of the case.
Once the powder choice has been made, the next step is to select and prepare the bullets. Bullet choice for subsonic shooting is much more critical than for conventional ammunition. Low velocity greatly reduces the projectile's rotational rate (RPM) and thus stability. Fast twist barrels are essential to successful subsonic loads. In .30 calibre this means a 1:10 twist or faster. A 1:10 twist will stabilize a 220 grain round-nose bullet at 950 fps.
Computer programs that calculate projectile stability are available free on the Internet. These calculators usually quote a Stability Factor (SF) of 1.3 for full stabilization in a conventional load. It has been found through testing that a SF in the range of 2.0 to 2.2 or greater is needed for a bullet to fully stabilize at subsonic velocity. This depends on how well the bullet is balanced with its center of mass. Lapua makes a special subsonic 200 grain .30 caliber bullet that is stable with a SF of only 1.36.
Bullet Stability Calculator
To prevent bullets sticking in a dirty bore, all conventional jacketed bullets used for subsonic loads must be lubricated. There are two primary methods of achieving proper lubrication. The traditional method is to dip the bullets in melted, refined animal fats (lard). The fat hardens as it cools, leaving a thin, slick surface on the bullet. The downside is this method is messy and slow.
The modern method is to simply use moly coated bullets. Moly is easy to get and can be applied to bullets by the hundreds in only a few hours. Moly coated bullets are also available direct from the factory. Moly coated bullets were used exclusively in testing for this article and the author has never had a coated bullet stick. The same cannot be said for uncoated bullets. As a side note, cast lead bullets are inherently lubricated and do not need to be coated.
One final note on bullets for subsonic loads. Never crimp and don't seat bullets into the lands. Bullets should be seated with lots of jump for safe operation.
The most radical modification required for safe subsonic ammunition comes in the case preparation. To take advantage of the magnum primer, the flash-hole should be drilled out to 9/64 inch (3.5mm) for a large-rifle primer. The enlarged flash hole allows as much of the primer flash as possible to reach the powder. This is especially important in full sized rifle cases. It also helps to prevent primer setback due to the low operating pressure of these rounds. DO NOT use subsonic modified cases for full power ammunition!
Now that all the components are gathered and properly prepared, we are ready to begin to develop ammunition. Unfortunately this is where things get difficult. There is almost no published subsonic load data anywhere. It is almost unheard of in any load manuals and it is fairly scarce on the Internet. A starting or reference point is needed to work from to avoid operating blind in the dark, which is not a good place to be when dealing with potentially explosive components.
CE Harris developed a fairly well known reduced power load known as
"The Load." Information on
"The Load" can be found fairly easily in an Internet search. To throw the reader a bone, here are the basics.
"The Load" can be used in any full sized rifle case without any modifications to the components. It consists of 13 grains of Red Dot (greater than 50 percent load density in .30-06 case) driving a lightweight bullet (125 grain in a .30 caliber) at medium velocity (approximately 1,600 fps).
The smart and experienced reloader should be able to extrapolate a safe starting load for heavy bullets from The Load. Keep in mind it is better to start a bit fast and work the velocities down slowly remembering NEVER to go below the 30 percent load density rule (It really should be a Law!)
When using a medium to small sized case, it is possible to compare internal volumes with other cases to find suitable starting data. For example the author discovered the .44 Magnum case (with a bullet seated) has almost the same internal volume as the 7.62x39mm case. All sorts of .44 Magnum load data exists for 180 to 220 grain bullets from which safe starting subsonic loads can be extrapolated.
Once the first test loads are assembled, it is time to head to the range and see how they work. A chronograph is essential to developing safe subsonic loads. Don't even try it if you don't have access to one. The target velocity to work towards 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.
During the initial testing period, it is important to start with a clean and lightly lubed bore to ensure bullets slip down the barrel easily. The bore should be lubed after every 2 or 3 shots to keep it clean and slippery. Velocities will vary somewhat due to the oil in the bore but at this point safety is more important than consistency. Make sure the bullet leaves the bore after every shot. Most of the time it is possible to hear the slow moving bullet hit the target but if need be physically check the bore. A stuck bullet will ruin your barrel and the day.
The first range session should be all about finding a safe load that meets the 950 fps requirement. It is certainly possible to go much slower than this but the danger of triggering SEE does not warrant the slight reduction in muzzle signature. Subsonics are very sensitive to barrel condition and fouling. Velocities can vary considerably depending on how clean the barrel is. Temperature can also have a great effect on subsonic muzzle velocity.
Once a safe load or two in the desired velocity range has been found, a second range session can be used to test the effects of reducing the bore lubing frequency as well as the consistency and trajectory of the load. Subsonic loads require the shooter pay attention at all times. A stuck bullet is always possible, with predictable results. Always remember a hangfire or delayed ignition signals impending SEE.
To easily distinguish modified cases (and subsonic ammunition) from conventional rounds, use a blue indelible felt marker to color either the case extractor groove or the case head. The blue case head (right) is more traditional and is still used by Lapua for their subsonic ammunition. The author prefers the blue extractor groove (below, right) because it is easier to see when handling ammunition and doesn't rub off as easily.
(marked cases - blue marking indicates subsonic load)
Once you become comfortable loading and shooting subsonic ammunition you will find that they are a lot of fun to shoot. There is hardly any recoil and muzzle signature is reduced to a slight pop. Shooting without hearing protection is comfortable and it is even possible to hear the bullet hit the target with a loud "Thwack!"
Five types of 7.62x39 subsonic ammunition ready for testing.
Rules for the safe operation of subsonic loads:
1. Drill Flashole to 3.5mm (9/64") to allow all the primer flash to enter the case and ignite the powder completely.
2. Use Magnum Primers for maximum primer flash and better powder ignition.
3. Lube all bullets. Moly is good, animal fat is better.
4. Lubricate the bore regularily while shooting.
5. Never crimp bullets into cases. Never seat bullets "into the lands."
6. Try not to use powder charges of less than 40% load density.
7. If load density is less than 40%, use tamping or fillers to keep the powder at the bottom of the case.
8. Make sure the bullet exits the bore after each shot.
9. Use only the fastest burning pistol powders - N310, N312, Bullseye, Clays, Titewad, HP38, Red Dot.
10. Any sort of slow ignition or hangfire is a warning of iminent Secondary Explosive Effect (SEE).
