By Mark White
Barrel Length and Overall Rifle Length
Most sniper rifles have barrels ranging from 24 to 30 inches in length. An effective suppressor needs from 8 to 12 inches of length in front of the muzzle in order to function properly. A 4 foot long rifle can easily become a 5 foot long rifle with the addition of a muzzle can, and this may be awkward in some situations. Most suppressed rifles have fairly short barrels in order to reduce the overall length. Expect to lose about 140 fps when cutting a 26 inch, .308 barrel down to 20 inches. As a practical matter, most high-powered rifle barrels are cut to between 16 and 18 inches. We have been taught from grade school that long barrels are much more accurate than short barrels, but this has no basis in fact. We personally find that an 18 inch barrel is a bit more accurate than a 26 inch barrel. The chamber, throat, crown and rifling are more important than barrel length. Subsonic rifle barrels may range from 8 to 12 inches in length. One does not need very much linear acceleration in order to reach a velocity of 1,000 fps with a .308 bullet. Privately owned rifles in the U.S. must have barrels over 16 inches in length, or they will require a $200 Federal Tax Stamp and registration in order to remain legal. A steel suppressor tube can be welded to a short barrel to avoid the tax and the hassle, as long as the overall length of the unit is beyond 16 inches. Soft solder or glue is not an acceptable alternative to welding. The intent of the BATF ruling is that the assembly must be permanent, and not easily altered. To clear up confusion, shotgun barrels must be over 18 inches in length. Again, rifle barrels must be over 16 inches long. This is of no concern to police and military where the organization or unit (not the individual) actually owns the weapons, although LE agencies are still required to register these with ATF.
If you have a choice of bullet weight it is useful to know that heavier bullets operate more efficiently (than light bullets) out of a short barrel. A .223 barrel should have a 1 in 7 inch twist in order to stabilize the heavier 69 and 80 grain bullets. A .308 barrel (shooting 180 and 200 grain bullets) is best served with a 1 in 10 inch twist. The common 1 in 12 and 1 in 14 inch .308 twists won’t stabilize any .308 bullet much heavier than the industry standard 168 grain, boat-tailed, hollow point, match projectile. If stability is a problem, round-nosed, flat-based bullets may be the answer. They are inherently more stable than sharply pointed, boat-tailed projectiles.
Subsonic bullets are not normally shot in combination with high-powered bullets. Because of softer recoil characteristics and less muzzle rise, a subsonic .308 can be expected to strike roughly 14 inches lower than a full-powered load at 100 yards. Again, the issues of cold shot zero and liability raise their ugly heads. During WW II, subsonic rifle bullets were sometimes loaded and fired backwards in suppressed rifles. This resulted in an increase in both accuracy and terminal effectiveness.
Tactical users who must use factory ammunition will be best served with the 168 grain, Limited Penetration (LP) round from Black Hills, unless they are shooting through steel or glass. The LP round duplicates other match rounds in accuracy and zero, but is designed to virtually explode upon impact, leaving no large fragments to exit the primary target and cause secondary injury to other individuals. A standard .308, 168 grain, match round has proven itself capable of penetrating over 40 layers of 1/2 inch sheetrock after exiting a primary target, a serious legal liability in the law-enforcement arena.
Suppressor Length, Volume and Profile
Design excellence aside, bigger is usually more effective. Volume can be achieved more effectively with diameter rather than length, but both are important. High-powered rifles require an exterior diameter of at least 1-1/2 inches, and a length in front of the muzzle of at least 7 to 10 inches. Subsonic rifles often require maximum suppression. Without a sonic boom it is possible to totally mask an event such as a gunshot. Since high-powered projectiles will always generate their own noise, there is little point in trying for extreme suppression. A good suppressor exists to make shooting comfortable without hearing protection, and to mask the location of the shooter. Greatly reduced recoil and an increase in practical accuracy are side benefits.
The traditional visual profile of a muzzle can carries a public relations stigma that goes back to 1934, and is not easily overcome. It is possible to disguise the profile of a slender muzzle can by extending the tube all the way back to the receiver. On a high-powered rifle this adds weight and expensive stock work. To some, the penalty of money and weight is worth the effort. The diameter of a Remington or Savage action is 1-3/8 inch, which seems to be the inside limit for a rifle suppressor. We had such a suppressor built (1-3/8 in diameter by 24 inches long) on a Savage .223 varmint rifle with a laminated wood stock. The disguise was extremely effective. A public relations stigma did not follow this weapon. We took it to firing ranges and gun shows and brandished it in public. Unlike the typical rifle with a muzzle can, no one appeared alarmed by its presence. Experts refused to believe it was anything but a bull-barreled target rifle, even after being told that it was suppressed. Unfortunately, the slim tube (extending a mere 8 inches in front of the muzzle) did not have the suppression rate that a normal 1 1/2 by 12 inch tube would have provided. A larger diameter action would allow a larger diameter tube to be installed without looking unusual. These actions are available, but they are more expensive. It is possible to have the tube larger in diameter than the action, but this looks unusual, thus the disguise is not as effective.
Gunfire noise is the most objectionable sound to the public at large. The louder it is, the more of a problem it creates in an urban area. A quieter sound is perceived as less lethal, and is therefore less objectionable. Where rifle fire must be used in an urban setting by law-enforcement personnel, a suppressor will greatly reduce the PR fallout, as long as it remains shielded from public view.
Typically, the hotter the suppressor gets with a single shot, the more effective it is. Full-auto fire with a suppressor will dramatically increase cyclic rate. It will also raise barrel temperatures considerably, because the hot gasses are trapped inside for a longer period of time. Full-auto fire is best kept to two or three-shot bursts. As a general rule we don’t expect machine guns to be very effective in a tactical scenario. Most perps will be behind cover by the third round. We are much more in favor of a single, carefully directed shot. We believe that accurate, effective, long distance tactical fire is more likely to occur with a bolt action rifle than with a bullet hose. The commotion associated with most machine guns is much more likely to draw attention than a single rifle shot, whether the firearms are suppressed or not.
The metals most commonly encountered in suppressors are chrome moly (usually 4130) steel, stainless steels, aluminum and titanium. Chrome moly steel is hard, tough and very durable. It takes non-reflective surfaces (Parkerizing) and holds paint very well. Paint is fast becoming the coating of choice, as it is corrosion-resistant, and can be changed (to camo) or easily renewed as situations warrant. Bake-on polymers can be cured in an oven at 350 degrees F. Some of these coatings are very tough indeed, and serve well in extended firings.
Aluminum is light in weight (about a third the weight of steel) but it is fragile and doesn’t take knocks, abuse and thread wear very well. Nor does it take heat well. It gets very soft and then fails and melts near 900 degrees F. By contrast, steel won’t melt until it reaches 2,700 degrees F.
Some aluminum cans are anodized, and then dyed black. The anodized coating looks good at first, but then gets beat-up and chipped. Aluminum does not take or hold paint very well, even after being sand blasted. Aluminum has a very high heat conductivity. This is a good property, because it will allow the material to rapidly absorb heat from the burned propellant gasses, reducing noise in the process. There is an old saying in the suppressor industry: “Put the fire out quickly, cool the gasses down.” Most aluminum is 6061-T6, which is much cheaper than steel, and moderately easy to machine, although it is sticky and tends to gall. End caps and baffles are usually machined out of 2024, which is soft and very easily cut. The alloy 7075 is sometimes used. It is hard, strong, more expensive, and abrasive to cut. Most aluminum will bend considerably before it finally breaks. The alloy 7075 cracks before it gives, and is not weldable. The alloys 6061 and 2024 are weldable, but most aluminum cans are threaded and glued together. When an aluminum can fails it usually does so at the root of a threaded joint, at the blast area.
The softness of aluminum makes it very prone to wear at contact points, such as the threaded joint where it is screwed or locked on to a barrel. Gas erosion can be severe in a high-powered rifle. Aluminum also has a high co-efficient of expansion, and this can cause problems with zero, or with a rapid loosening of parts. It is so soft that axial alignment may eventually become a problem as threads get beaten loose and sloppy. Cast aluminum is very porous and weak, and should not be used.
Stainless steel has nickel and chrome alloyed in with the steel. It is more corrosion resistant than steel or aluminum. Stainless holds paint poorly, and also has a high coefficient of expansion. Stainless comes in many grades and hardnesses. The harder grades can be brittle. The softer grades are subject to thread wear and deformation from battering. Stainless is expensive and hard to machine. It is not available in the variety of sizes that one finds with aluminum and steel. Stainless has a fair degree of conductivity. It is not as reliable as steel. When a stainless can fails it usually does so along a seam, or at the root of a threaded joint in the blast area.
The commonly used type 304L series of stainless is dead soft, very corrosion resistant, easy to machine and welds beautifully. Its downfall is that it is easily deformed. If a can made of 304L is dropped or impacted in shipping or deployment it may easily be deformed, and this may affect axial alignment. Other commonly used stainless alloys, such as 316 and 321 tend to be harder and more resilient, but they are also much more difficult to machine and weld. There are literally hundreds of stainless alloys available, and they may have very different characteristics. Those used for some rifle barrels have high percentages of sulfur and lead, which improves machinability while decreasing wearability. The ideal stainless alloy would posses the ductility, machinability, resilience and weldability of 4130, chrome moly steel, yet be susceptible to corrosion.
Titanium is about half the weight of steel, has a very poor conductivity, is very expensive, is highly resistant to corrosion, and is almost as strong as steel. It is very difficult to machine. It destroys cutting tools because its abrasive nature combines with its poor conductivity to produce high heat buildup in cutting tools, softening their edges. Titanium takes and holds paint fairly well. It is about as shiny as stainless, but is a little darker in color. Titanium’s light weight and strength are a plus. Extremely high cost and poor conductivity are a minus. There are several titanium alloys available, the most common of which is 3, 2.5 (pronounced three two five), containing 3 % aluminum, 2 1/2 % vanadium, and 94 1/2 % titanium, which is used in high-end bicycle frames. The alloy 6A4V90T is sometimes used in receivers and barrels. The bore life of a titanium barrel is not especially good.
Steel, stainless steel and titanium are very weldable using the TIG (tungsten, inert gas) process. Aluminum is also weldable, but it takes a high degree of skill and experience to do so effectively. Only steel and stainless steel are able to be welded to each other in a meaningful way.
Threading and Alignment
Suppressor manufacturers fall into two camps – Threaders and Welders. Neither likes the other, and both think that their own methods are vastly superior. Threading greatly weakens the tube. Welding is strong and permanent, but distorts the metal. Just as there is a constant and vigorous search for better and more complex baffles, so too does the controversy between welders and threaders rage.
There is usually a demand for a wide variety of suppressors, so individual manufacturers stock a variety of parts for different models, most of which have been turned out on CNC or automatic screw machines, in limited runs. The parts are kept in bins, and usually consist of a main tube or body (which is the registered, serially numbered part in the U.S.), a front end cap, a rear end cap, and a baffle stack – which may or may not be sequentially important. If the baffle stack is sequential the larger spaces are usually towards the rear, near the barrel’s muzzle, while the smaller spaces are probably in the front. So many cans have been taken apart by incompetents, and reassembled incorrectly, that the industry trend has been towards sealed units that can be cleaned by immersion in a solvent. Design rip-offs are common in the industry, thus welding and sealants are also used to mitigate intellectual thievery. Baffle design, optimal spacing and proportion are critical to performance. Patents abound, but they afford little protection in foreign countries. Often, patent drawings do nothing more than afford competitors baffle designs that they would otherwise have to purchase and destroy suppressors in order to obtain.
A can with a single-point mount relies on the rear end cap for all of its axial and angular alignment. The rear end cap was probably made on an automatic screw machine, and bored and threaded to take a barrel at the same time. It is critical to the alignment procedure that this was done with extreme accuracy, as a tiny amount of angular or axial misalignment can result in severe misalignment (and possible baffle contact) in a 10 inch long can. The best way to bore and thread a rear end cap is to screw and glue (or weld) it into its suppressor tube first, and then place the entire unit in a lathe for the remainder of the machining. Few bother to do this, however, because it is much easier to take finished parts out of bins and assemble them. Line boring and threading in a lathe might mean having to refinish already completed parts. We have seen a 7 inch long can from a prominent manufacturer that had 3 degrees of angular misalignment after it was mounted on its dedicated barrel. This may not sound like much, but try to remember the sometimes-close tolerances between bullet path and baffles.
If the rear end cap had been welded instead of threaded on, the chances are better that the unit had been bored and threaded on a lathe after assembly. Welding induces distortion as the liquid metal cools, solidifies and shrinks, so it is nearly impossible to successfully thread the bore of a rear end cap before welding. Almost all of the angular and axial alignment problems we see today are related to a single-point mount on a rear end cap that has been improperly machined prior to being assembled. Again, tubes that have threaded rear end caps are prone to fatigue and a possible massive failure at the root of the last inside thread, at the blast chamber. Pressure is low near the front of the can; thus we rarely see a failure at this point, unless there is baffle contact and bullet tumbling.
Most two-point mounts have the threaded portion (commonly called “the nut” or “the spider”) located in a more central portion of the suppressor tube. This threaded portion is very important, as it holds the entire can in place – usually by pulling it tightly against the rear end cap. There are many methods of holding the nut in place, and none of them are without their problems: A snap ring may be inserted in a groove in the center of the tube, but this groove weakens the tube near the blast chamber. The nut must also be pinned in place, or it will rotate. The nut can be plug-welded through holes drilled in the tube, but the welding process slightly distorts (bends) the tube, even if 4 or 6 welds are placed in direct opposition to each other. The welded or threaded rear end cap can push a thin section of tubing against the nut, but that nut must still be pinned or glued in place or it will rotate. Lastly, the nut can be silver soldered or brazed in place, but the area is tough to see through smoke and fume inside the tube, hence it is difficult to be sure that a proper bond has been achieved. The soldering process may distort the tube. One must be careful to boil out the tube in water afterwards, to remove corrosive salts left by the soldering flux.
The barrel which mates to a suppressor must be turned in a lathe. Threads are best turned with a cutting tool as the barrel rotates between centers, but I have also seen satisfactory results obtained with a die-holding fixture in a carriage or tailstock. Machine threading on a lathe with a single point cutting tool is often called single pointing, which is not to be confused with a single-point mount. It is felt in the industry that single point threads are the most accurate. Rolled threads are the strongest, as they are forged during the process of rolling between two dies.
Few barrels are either straight or symmetrical, and this is another possible source for angular or axial misalignment. A practiced eye can spot a barrel with a crooked bore. If the muzzle is clear and open, one can peer down the headstock as the barrel spins in a lathe to get a good idea about how true the bore is. Short, thick barrels are easier to deal with than long, thin ones. Barrels which are fluted are often bent during the fluting process if they are not properly supported and frequently rotated. Fluting has ruined many otherwise perfectly good barrels. Most fluting is purely decorative in nature, and normally performs no useful function; no matter what manufacturers claims are made to the contrary.
Many schemes have been devised to seal a rear end cap where it joins its barrel. A tight mechanical seal is usually effective, but sometimes rubber or silicone (high heat) O-rings (or sealants like pipe dope) are used as a backup, in the event that the suppressor loosens as it is being used. For the sake of reliability and cold shot accuracy, it is critical that a suppressor not loosen on its barrel.
If a bullet path is not perfectly straight the holes in suppressor baffles will have to be enlarged to accommodate. Due to angular dispersion, those baffles nearest the muzzle can have holes which are smaller. We live in a real world, not a theoretical one. Most barrels have bent bores. Most bores do not lie in the true center of a barrel. Most suppressors are not perfectly aligned. Threads wear. Welding distorts. That’s why we have tolerances, and sometimes situations require that those tolerances be increased. Tight baffle holes are more important near the rear, where high-pressure gas exists, than they are near the front. Asymmetrical baffles usually work best when they line up parallel to each other. Some manufacturers have a method of holding those baffles in proper alignment. Some just drop them into a tube and hope that they stay aligned.
Front end caps are either welded in place, or screwed in. If the manufacturer wants to be able to get back inside the can at a later date he will use a weak glue or ISPBA (intermediate-strength, proprietary bonding agent – also known as Blue Loctite). If he wants the end caps to stay, he will use a stronger glue, or HSPBA (higher-strength, proprietary bonding agent – IE Red Loctite). Aluminum does not glue well, even with 2-ton psi epoxy. If the can gets hot from rapid use most adhesives will loosen and eventually fail. Even secret, high-strength proprietary bonding agents will eventually give up and work loose at the rear end cap, where most of the heat and shock are concentrated.
It is difficult to summarize a complex topic in its entirety. Those who use high-powered sniper rifles have every right to expect those rifles to be dependably accurate. A properly suppressed rifle must be constructed with a serious commitment to both suppression and reliable accuracy from the outset. This usually means a steel or stainless steel silencer mounted to a short, heavy barrel with a robust, two-point mount. One can expect both barrel and silencer to exact a combined penalty of at least four or five pounds in weight, and an extra six inches in overall length.
Continued and diligent practice on a regular basis are vital to a mission. Practice should be held at night and during inclement weather, as well as on warm, sunny days. Only a few rounds need be expended, but they should all be accurately and carefully delivered. One should always look through the bore to ensure that it is clear before a callout or deployment. Wasps and other insects have a nasty habit of building mud nests in inconvenient places, and a plug of mud can be disastrous to bullet placement. Always store the weapon muzzle-down. The suppressed rifle must remain dedicated to suppressed fire only. To do otherwise compromises cold shot reliability. The benefits of a suppressed system include a low profile, relative obscurity, increased accuracy, decreased recoil, greater stability, and a lower likelihood of detection in the field.
|This article first appeared in Small Arms Review V1N8 (May 1998)|