Subsonic Info + Tools

Below you will find information I am sure you will find useful. Make sure you keep checking this area as we update it often.


Speed of Sound (ft/s)

  • λ = 49.06 x √(T + 459)
  • λ is the velocity of sound in feet per second and T is measured in degrees Fahrenheit
  • T (°F) / λ (ft/s)
  • 0 / 1051
  • 32 / 1087
  • 44 / 1100
  • 50 / 1107
  • 60 / 1118
  • 70 / 1128
  • 80 / 1139
  • 90 / 1150
  • 100 / 1160

First Round Pop

It doesn't take long, particularly with subsonic ammunition, for the operator to notice that the first round fired is considerably louder than subsequent rounds. It is only 2-3 decibels on a good sound meter, but since decibels are logarithmic rather than linear, to the human ear, this is a lot. If there is a "cooling period" of more than a few minutes, FRP rears its ugly head again. Operationally, this can be a very undesirable thing, so here are two methods of combating this phenomenon.

1. If your suppresser is of the "Quick Detach" type, perhaps the easiest and most effective method is removing the can and spitting in the rear (blast chamber) end.

2. A more technological solution is canned Nitrogen gas. Available at any computer store outlet, this is the stuff with which you dust your monitor screen. The method is to chamber a cartridge to seal the rear of the barrel, and BEING CAREFUL WHERE YOU POINT THE LOADED WEAPON fill the bore and suppresser with the Nitrogen gas. A squirt of several seconds does the job. Put a piece of tape over the muzzle to hold the gas in. This method is good for several hours, until the Nitrogen slowly leaks out the back around the unexpanded cartridge case.

3. As well as the phenomenon of first round pop, the first round out of a cold barrel is often slower than succeeding shots due to energy - that would otherwise be used to propel the projectile, being soaked up and used to heat the steel barrel. This is exacerbated by a long barrel, as discussed in the article "Subsonics Do Not Like Long Barrels".

Instructions for Subsonic Rifle Ammunition

1. Always ensure that the weapon is clean before firing subsonic rifle ammunition. A dirty bore creates a dangerous situation.

2. Always ensure that the previous projectile exited the bore before loading or firing the next round. If in doubt, stop and inspect the bore.

3. Always treat subsonic rifle ammunition and the weapons it is in with the same respect given any other ammunition and weapon. Just because it is quiet does not mean that it is not as dangerous as any other weapon system.

These instructions are necessary because subsonic rifle ammunition is not a "one size fits all" proposition due to variances in bore size and comparative smoothness or roughness, and because chamber dimensions are not all the same. While reasonable care is exercised in the production of this ammunition and it functions in most weapons satisfactorily, it is the end user's responsibility to ensure that his particular weapon works safely with this specialized ammunition.

Proper Use of Subsonic Ammunition

There are several problems associated with the use of subsonic ammunition in gas operated, semi- and full automatic weapons, and even occasionally in bolt-action weapons (see "phosphating").

The first and most important is that since the action is closed by means of an operating spring, the cartridge may not be seated fully forward in the chamber. This commonly causes a failure of the cartridge case to obdurate (seal) and the propellant gasses,especially in the very tapered 5.56mm cartridge. Instead of pushing the projectile out of the bore, flow back by the cartridge case, usually sticking the projectile an inch or so ahead of the chamber in the bore. The danger of this condition is obvious, and it is therefore of utmost importance that the operator pay attention to each round, ensuring that the projectile has left the bore - indicated by impact sound. The indicator of failure to obdurate is a slight but audible "hissing" sound as the propellant gasses flow back by the cartridge case. Also, the cartridge case will have a "smoked" appearance at the forward edge, back to the shoulder area. The very tapered profile of the 5.56 cartridge case lends itself to this failure mode, although it has occurred in some chambers, usually with an out-of-spec throat in .308/7.62 as well.

The correct procedure to avoid this pitfall is to give the Forward Assist a sharp rap prior to firing each cartridge. This ensures that all bolt/carrier parts are fully forward as well as the chambered cartridge. Whilst the AR-family of weapons does have a safety cam on the firing pin, this safety is designed to prevent firing of the weapon with the bolt in the unlocked position (with high velocity ammunition) and does NOT prevent firing with the parts not fully forward and fully locked. Another way to ensure full chambering, particularly in those weapons which are not provided with the Forward Assist is to short-load the magazine by several rounds, thus lessening the amount of force spent by the operating spring/buffer in stripping a round from the magazine and increasing the amount of force it can spend on chambering the round.

The second undesirable condition present in gas-operated firearms is the presence of the gas bleed hole in the bore. While this does not prevent the use of subsonic ammunition, since SSA uses much less propellant than high velocity ammunition, the presence of a bleed-off does tend to produce more velocity variation and thus vertical stringing of the shot group than that found in a solid bore.

The third and perhaps most relevant problem we have found in gas-operated firearms is the presence of phosphate particles inside the bore of both gas-operated and other action types which have received an outer phosphate finish to the barrel. This problem can cause projectiles to fail to exit ( Think of the subsonic projectile hitting a grappling hook with a parachute attached on its way down the bore) This condition has become more prevalent in the last few years. It is caused at barrel manufacture by the phosphating processor failing to seal the bore whilst the phosphate coatings are applied to the outer barrel surfaces, and has been observed in some of the most and least expensive of firearms. It is plainly visible through a good quality borescope. Shooting high velocity ammunition will not remove these particles. EBR is currently working on a relatively inexpensive solution, and in the future will warrant the use of its ammunition only in barrels to which this solution has been applied (thus ensuring the absence of phosphate particles in the bore. The cure has the added advantage of assisting in good accuracy, even with the use of high-velocity ammunition.

Finally, subsonic rifle ammunition will not work in a firearm which has a fluted chamber (NOT barrel). This fluted chambering is typically found in delayed-blowback firearms and is put there to purposely direct propellant gasses back past the fired case to help "float" it out of the chamber and prevent case sticking in high-velocity ammunition. Obviously, it can cause subsonic ammunition to fail to obdurate (seal the chamber) causing a failure of the projectile to exit the muzzle. Subsonic rifle ammunition SHOULD NOT be used in this type of firearm for this reason.

The above conditions do not necessarily prevent the use of subsonic ammunition in a gas gun, but do impart on the operator the responsibility of carefully monitoring the effect of these conditions in his/her particular firearm. As always, any ammunition carried needs to be tested in the firearm to be carried prior to any operational use. This is just good common sense.

How to Verify Stability

Will I get baffle strikes with this ammunition?

The method to verify stability is to place a piece of heavy cardboard 10’ or so from the muzzle and shoot several rounds of the ammunition in question through it with the suppresser off. If the hole is perfectly round, you are stable. If any are at all oblong, you run the risk of baffle strikes. This is true of any ammunition in any weapon system.

Why Use Phosphate Removal for Subsonic Ammo

Phosphate in Your Rifle Bore & Subsonic Ammunition

With the advent of many more Tactical rifles on the market the problem with some of these weapons of Phosphate particles in the bore has appeared.This is occurring in both very expensive, quality firearms as well as the less expensive ones.

What happens is that when the barrel or weapon manufacturer applies the Phosphate coating (that "rough" grey or black military finish) to the exterior, the muzzle and/or chamber end is not being completely sealed, allowing some of the Phosphate particles to enter the bore and adhere to the chrome lining or barrel steel. The result when any Subsonic ammunition is fired is predictable since the Phosphate particles act as a very effective brake, radically slowing and sometimes stopping the projectile in the bore; an inconvenient and potentially dangerous situation.

The Good News™ is that the particles are mechanically rather than chemically adhered to the barrel, although it is a fairly strong bond and just shooting high velocity ammunition through it will not remove the Phosphate. The Solution: Remove the Phosphate particles. Fortunately, this only has to be done once and then the tube - given that it has the correct chamber dimensions, etc. – will be suitable for use with subsonic ammunition from then on. This can be done via the use of EBR Phosphate Removal System, available through EBR, Inc. on our website

At last EBR is able to provide clients with a solution to the problem of phosphate "overspray" in the bores of Tactical weapons.

We have noticed in the recent past an upturn of the "stuck in bore" syndrome from clients utilizing subsonic ammunition. Whilst the first reaction is often "the ammo didn't have any powder in it", this unlikely since EBR uses CAMDEX computerized, automated loading machines which stop immediately whenever a round has too much or too little propellant in it. So while anything is possible this is not very likely.

So we dug deeper and found that with the great increase in the number of "Tactical" weapons in use, there was a corresponding increase in the use of phosphate firearm finishes. This is the dark grey or black gritty-feeling finish found on most M16/M4 type weapons. And now almost every gun company produces some version or another of a Tactical firearm, most all of which have the phosphate finish.

Unfortunately, not all manufacturers are being careful about sealing the bore and chamber of the barrel when applying this finish, resulting in phosphate particles in the bore of the weapon. Some examples we've seen were so bad they were coated from chamber to muzzle and produced appalling accuracy even with high velocity ammo. They can be plainly seen with a good-quality borescope.

This condition only affects high velocity ammunition slightly (being detrimental to accuracy), but it affects subsonic ammunition dramatically. Think of the subsonic bullet hitting a grappling hook with a parachute attached on the way down the bore. Just shooting high velocity ammunition does not remove these tenacious little critters, but our phosphate remover does.

And the Good News™ is that it only has to be done one time on a barrel to make it compatible with all subsonic ammunition from then on…provided of course it was a decent barrel to begin with. We only have one M4 barrel in our vault (new) from a major manufacturer that simply will never shoot subsonic ammunition and doesn't do so great with high velocity either…that is because of the very deep tool marks made when the rifling was cut; it will just never be a good barrel, period.

Just follow the included instructions CAREFULLY, and improve the performance of your Tactical weapon. Just $18 delivered to your door with enough to treat ~3 guns or one REALLY bad one.

Phosphate Removal Solution


You must use a cleaning rod guide that uses o-rings, during the parkerization removal process. The provided solution will remove parkerization on the inside of the bore, as well as the outside if you spill any on the outside finish.

  • EBR WILL not be responsible for your use of this product if it removes your firearms finish.
  • DO NOT
  • use this solution on a barrel that is hotter than 105 F.
  • use this solution on a chrome bore that has been extensively fired, or fired full auto.


1. Use a chamber brush with a low speed drill to clean the chamber and throat of the firearm. This method will not work in the bore, as the rifling prevents the brush from contacting all of the parkerization stuck in the rifling grooves.

2. Clean bore with solvent and JB compound or equivalent. Run alternating wet and dry patches through the bore until they come out clean.

3. Clean bore with more patches soaked in acetone. This will remove oil and grease from the bore to allow the solution to directly attack the parkerization.

4. Clean the outside of the muzzle of the firearm, and tape a heavy plastic bag with a hole in it for the muzzle, over the muzzle using large amounts of tightly stretched electrical tape or the equivalent. Make sure the tape and bag covers everything where you don't want the finish removed. Tape another plastic bag over the muzzle to prevent the splatter of the parkerization removal solution when the cleaning brush exits the muzzle.

5. Use tape, plastic and/or furniture wax on the action/receiver to protect that finish as well.

6. Add at least 1 Table spoon of baking soda to a cup (8 oz) of distilled water. Heat to at least 110 F and stir to ensure the baking soda is dissolved. You will use this later to neutralize the park removal solution from the bore as well for any drips or spills.

7. Mount the firearm so that the muzzle is pointed down from horizontal by at least 15 degrees. You do not want to drip the solution into your action.

8. Wrap a cleaning patch on a brush, add enough of the cleaning solution so that the patch is visibly wet, but not in danger of dripping. Using short strokes of 2-3 inches, slowly work the brush up the bore to the muzzle. This should take a full 2 minutes.

9. Do not change the patch, add sufficient solution to re-wet the patch and brush combination, and repeat for a total of 3 times.

10. Using a new patch wetted with solution, use long strokes, 5-10" at a time, work your way down the barrel.

11. Then let the barrel soak.

12. Use the soak times guide (below)

13. Using a patch or mop soaked in baking soda solution, mop out the bore a least 5 times. You are neutralizing the parkerization removal solution. You may put the muzzle of the firearm in the baking soda solution and using a tight fitting patch, suck up that solution into the bore. This is optional, but does speed up the process of cleaning.

14. If your barrel has a gas port, make sure the baking soda solution flushes out that gas port and/or tube.

15. If you have dripped the parkerization removal solution anywhere on the gun, you can use the baking soda solution to neutralize any drips or spills.

16. Next, clean with a brush and conventional solvent until patches come out clean.

17. Clean with JB compound or equivalent.

Soak Time Guide

  • Do not use if the barrel is > 105 F
  • <70 F | 15 min
  • 70-85 F | 12 min
  • 85-95 F | 8 min
  • 95-105 F | 4 min

Subsonic Rounds and Long Barrels

Subsonics Do Not Like Long Barrels!

We just had another customer ask about subsonic use, etc. Thought I'd share some info. This Arkansas cop was having trouble with his gun and our ammo. Turns out it was the same phenomenon we discussed years ago with Al about the first shot being much slower due to the cold steel soaking up energy. His gun/EBR ammo chrono'ed about 650 on the first shot, then in the high 900's to mid 1000's on all succeeding shots.

I noticed his barrel was WAY long, and I think this exacerbates the problem, so we're having it cut by a competent 'smith (Jared Joplin of Patriot Arms / 706-367-8881) and rethreaded to 20" at his request.

Pretty sure this will cure the problem from the testing evidence; I'll let you know. If not, I just bought another barrel


Got the customer's gun back, shortened to 20". The very first round from the same box of EBR PBS's chronographed at 1046 ft./sec. with no warming shots fired. Mean Average for the 5-shot group was 1021 ft./sec. - PERFECT!

The moral of this story is that if you intend to shoot subsonic .308 Rifle ammunition in your platform, it is best to not only rebarrel the gun to put a better tube on it, but to also have it cut to no longer than 20". For those who think this ruins the Long Range capability of the weapon, I say "HOGWASH". We regularly shoot to 900 yards with the 16" Wraith with excellent results on calm days. It has been proven beyond the shadow of the slightest doubt that a short, stiff tube is more accurate than a long whippy one, *especially* if you put a weight (suppresser) out on the end of it.

While we're on this subject, I do not consider the .308/7.62mm to be better than a 750-yd. weapon IN REAL LIFE. Yes we shoot them much further, but if there is any wind at all...especially if it is gusty...the results get real "iffy". If you need more than the 750-800 yards, it's Time to consider the .338 Lapua or Mr. Fifty so as to be able to buck the wind.


Subsonic Ammo and Milldots

A Most Useful Subsonic Tool!

If your rifle is zeroed for high velocity at 100 yards and has an optic with a Mil-Dot reticle, use the 3rd Mil-dot down as an aimpoint at 50 yards, and the fourth Mil-dot down for an aiming point at 100 yards for our subsonic ammunition.

This gives you a quick, tactical "aim-shoot" solution for most situations you will encounter without having to fuss with your scope.

This differs somewhat from platform to platform, but once you get used to your gun/scope, it will be within an inch of spot-on.

Greenhill Formula for Rifling Twists

The Time-honored Greenhill formula was originally used for determining twist rates in the new rifled artillery in the 19th Century.. It is important to note that it is projectile *length*, not *weight* which is the determining factor when considering the best twist rate for your intended use. Weight can vary for a specific bullet length from use of different material and also is determined to some degree by projectile shape. Often long-range match shooters utilizing low drag or VLD projectiles will voice a preference for "lazy" twists; i.e. just enough to stabilize the bullet. The requirements of a Shooter who is utilizing both high velocity and Subsonic ammunition are quite different.

When utilizing subsonic ammunition there is another factor to take into consideration. The projectile not only has only (about) 1/3 of the forward velocity of standard ammunition in Rifle calibers, it also has 1/3 of the *rotational* velocity...think "RPM's". This makes for a less gyroscopically-stable projectile, so a faster rate of twist is indicated than by simply applying Greenhill alone. This wisdom was imparted to me in a conversation with the great barrel-maker Boots Obermeyer. It makes perfect sense, and doubly so coming from him, so we'll take it as Gospel.

Greenhill Formula For Rifling Twists

T x B= 150 x Sqrt (Density of Lead/Density of Bullet)

T=Twist / B= Bullet length

Both units have to be in 'Calibers', density portion is optional

Determining twist rate for a projectile (varies by shape)

Using the a 190 gr Sierra MatchKing bullet #2210M (.,30 cal) as an example. If we measure the bullet's length it is 1.375" long .

B=Bullet length in *Calibers* (Length/Diameter), hence 1.375/0.308= 4.46 *calibers* long

From formula we have T=150/4.46=33.6. This figure is now in *calibers*, so we convert the twist from calibers to inches. So T=33.6 x 0.308=10.34".

This is why most commercial rifles in .300 Win. Magnum come with a 1:10" twist barrel.

One can go further and get involved in formulas for density, but for most conventional lead/gilding metal projectiles, the above will suffice. Note that this is for a normal High Velocity loading. Subsonics generally require a somewhat faster twist rate. If one plans to use both High Velocity and Subsonic loads in the same platform, a good compromise which works acceptably for both must be found. If the platform is a "dedicated" gun in which subsonic ammunition alone will be used, then a faster rate can be utilized in order to be able to accurately shoot longer (and thus heavier) projectiles subsonically and with good accuracy.


Twist rates for 7.62mm/.308:

High Velocity only: 1:11-1/8" to 1:12"

Both: 1:10"

Subsonic ONLY: 1:8"

Bear in mind that "dedicated" back at the shop might not be "dedicated" in the field. Most knowledgeable end users prefer to have the option of utilizing both loads so as to be able to address changing tactical requirements on the spot.