Pressure signs with suppressor

[CMP70306]

So can someone explain to me how a suppressor is increasing the pressure to higher levels when the peak pressure is achieved long before the bullet is anywhere near the suppressor? By the time the bullet gets all the way down to the end of the barrel the pressure in the barrel is substantially lower than the 55k+ psi peak pressure so as a result any back pressure from the suppressor would be substantially lower.

My guess is one of two things happening, the first is that blow back gases are increasing the chamber fouling resulting in the case not gripping the walls as tight during firing. This could resulting in an increase in bolt thrust which would show up as ejector marks since the brass is pushed onto the bolt face with more force as it breaks free earlier in the pressure cycle.

The second is that the prolonged back pressure due to the can maintaining some of the pressure after the bullet has left the barrel could result in the brass being pushed back against the bolt face after it is no longer expanded to the point of pushing against the chamber walls.

Essentially in both cases it isn’t that the suppressor is causing higher chamber pressures and an unsafe load, it’s that it is causing false pressure signs in the brass after the bullet has left the barrel.

 

[Shortschaf]

So can someone explain to me how a suppressor is increasing the pressure to higher levels when the peak pressure is achieved long before the bullet is anywhere near the suppressor? By the time the bullet gets all the way down to the end of the barrel the pressure in the barrel is substantially lower than the 55k+ psi peak pressure so as a result any back pressure from the suppressor would be substantially lower.

My guess is one of two things happening, the first is that blow back gases are increasing the chamber fouling resulting in the case not gripping the walls as tight during firing. This could resulting in an increase in bolt thrust which would show up as ejector marks since the brass is pushed onto the bolt face with more force as it breaks free earlier in the pressure cycle.

The second is that the prolonged back pressure due to the can maintaining some of the pressure after the bullet has left the barrel could result in the brass being pushed back against the bolt face after it is no longer expanded to the point of pushing against the chamber walls.

Essentially in both cases it isn’t that the suppressor is causing higher chamber pressures and an unsafe load, it’s that it is causing false pressure signs in the brass after the bullet has left the barrel.

I don't know exactly why, but I personally don't care.

To address the rest of your post, pressure is pressure. Nothing false about it.
Lightening the load is still the resolution

 

[EastHumboldt]

Increased velocity coupled with a hard bolt lift. Only occurs when the can is on. Goes away when the can is off.

Sounds like real pressure, sounds like the can is causing it. Either reduce the load until the pressure signs go away and the extra velocity, or learn to enjoy living dangerously.

 

[Southern Lights]

So can someone explain to me how a suppressor is increasing the pressure to higher levels when the peak pressure is achieved long before the bullet is anywhere near the suppressor? By the time the bullet gets all the way down to the end of the barrel the pressure in the barrel is substantially lower than the 55k+ psi peak pressure so as a result any back pressure from the suppressor would be substantially lower.

My guess is one of two things happening, the first is that blow back gases are increasing the chamber fouling resulting in the case not gripping the walls as tight during firing. This could resulting in an increase in bolt thrust which would show up as ejector marks since the brass is pushed onto the bolt face with more force as it breaks free earlier in the pressure cycle.

The second is that the prolonged back pressure due to the can maintaining some of the pressure after the bullet has left the barrel could result in the brass being pushed back against the bolt face after it is no longer expanded to the point of pushing against the chamber walls.

Essentially in both cases it isn’t that the suppressor is causing higher chamber pressures and an unsafe load, it’s that it is causing false pressure signs in the brass after the bullet has left the barrel.

This just happened to my Blaser R8 in 270 running an A-Tec H2 suppressor and factory Superperformance ammo. The rifle never has blown a primer before, but did when this suppressor was used. I removed all but one baffle and still did it. I swapped it for an ASE Utra SL7 and the problem stopped.

Looking at the case they had ejector marks or blown primers. They were indistinguishable from pressure signs from a load that is too hot during reloading. I have never blown a primer in this rifle prior to this or come anywhere near close. The rifle was clearly unhappy.

The one thing is certain for me: The addition of the suppressor absolutely increased pressures enough to cause the problem. The problem is repeatable and never happened bare rifle or with the other suppressor.

Other people I know that use H2s don't have the issue. I feel it will be completely rifle/load/calibre specific for each suppressor. But clearly pressures are increased when running a suppressor of any type in my view.

 

[CMP70306]

I don't know exactly why, but I personally don't care.

To address the rest of your post, pressure is pressure. Nothing false about it.
Lightening the load is still the resolution

Below is the pressure curve of a bullet traveling down the barrel. By the time it reaches the muzzle, where the suppressor begins to have any effect, the chamber pressure is down to 10,000 psi, less than one 6th of the initial chamber pressure.

So you’re telling me that the suppressor somehow increases the pressure to higher than the initial chamber pressure after the bullet has left the barrel?



What’s more likely is that it’s either coating the inside of the chamber in carbon causing the cases to slip prematurely under pressure or it is holding the pressure curve longer causing the same result.

 

[Southern Lights]

So you’re telling me that the suppressor somehow increases the pressure to higher than the initial chamber pressure after the bullet has left the barrel?

None of these charts take into account fitting a suppressor that is impinging gasses leaving the barrel. They assume bare muzzle.

All I know is this:

- When I put on one suppressor on a rifle it blew out primers and made clear ejection marks on brass.
- When no suppressor is on the rifle it never does this.
- When I put on another brand of suppressor on the same rifle and same ammo it didn't do this.
- When I put the problem suppressor back on, it did it again.

So regardless of what a chart shows, empirical data is that suppressors are driving pressures in the rifle higher.

 

[Shortschaf]

So you’re telling me that the suppressor somehow increases the pressure to higher than the initial chamber pressure after the bullet has left the barrel?.

I'm saying that the pressure you are seeing is real, and that the answer for alleving that pressure is precisely the same as if it were a hot load on an unsuppressed rifle.

So I am not suggesting that I understand WHY. I simply don't care what the charts or books show if I am seeing pressure in real life

 

[CMP70306]

None of these charts take into account fitting a suppressor that is impinging gasses leaving the barrel. They assume bare muzzle.

All I know is this:

- When I put on one suppressor on a rifle it blew out primers and made clear ejection marks on brass.
- When no suppressor is on the rifle it never does this.
- When I put on another brand of suppressor on the same rifle and same ammo it didn't do this.
- When I put the problem suppressor back on, it did it again.

So regardless of what a chart shows, empirical data is that suppressors are driving pressures in the rifle higher.

But it isn’t driving the peak chamber pressure higher, peak chamber pressure is achieved when the bullet hits the rifling long before the gas has any possibility of making it down to the suppressor to have any effect. And if it isn’t affecting chamber pressure then

What is more likely is that the chamber pressure drops to near zero when the bullet exits the barrel into the suppressor. At that point the pressure wave caused by the gases trapped in the suppressor travels back down the barrel and into the chamber.

With the chamber pressure reduced the case is no longer expanded and stuck to the walls of the chamber or only lightly so. As a result the gases are push the case backwards into the bolt face with several thousand PSI of pressure. This would cause the same types of pressure signs seen by an over pressured case such as ejector marks and flattened or blown primers.

The same issue can happen with lubed cases, they can show pressure signs with safe loads due to the case slipping in the chamber and putting excess force onto the bolt face.

Now for the specific reasoning as to what causes it in some cases and not others it’s most likely dependent upon a multitude of factors. Since Superformance develops its higher velocities by lengthening the pressure curve that would make it much more susceptible to the above scenario as the pressure at the muzzle would be higher due to the progressive burn of the propellant.

Looking at the A-Tec H2 with its perforated hole design and a website that specifically states that it is for hunting rifles only implies that it was not designed to work on Semi Auto rifles due to high back pressure. That could be the reason you see it with that suppressor and Superformance ammo but not the other suppressor.

 

[Southern Lights]

Looking at the A-Tec H2 with its perforated hole design and a website that specifically states that it is for hunting rifles only implies that it was not designed to work on Semi Auto rifles due to high back pressure. That could be the reason you see it with that suppressor and Superformance ammo but not the other suppressor.

The Blaser is a straight pull bolt action. It is not a semi-auto. The H2 in that calibre and that load blew the primers. I don't even blame the suppressor, it's just that suppressors can cause this issue.

Why? I don't know but it looks and acts like overpressure. Complete with blown primer, ejector marks, and difficult to operate bolt handle. It is a dangerous situation.

But, who am I to believe? The rifle with blown primers, ejector marks and hard to work bolt handle? Or, a random pressure chart generated with who knows what parameters posted on the Internet?

 

[General RE LEE]

I’d like to tag on this thread but I get this same issue with my Banish Backcountry. I get dirty chamber and pressure signs on occasion. This is with loads that are not remotely hot. It only happens on rounds later in a string. He first few shots on a cool bore no issue. Once the can starts getting hot, I get pressure signs primarily stiff bolt lift.

 

[goathunter]

None of these charts take into account fitting a suppressor that is impinging gasses leaving the barrel. They assume bare muzzle.

All I know is this:

- When I put on one suppressor on a rifle it blew out primers and made clear ejection marks on brass.
- When no suppressor is on the rifle it never does this.
- When I put on another brand of suppressor on the same rifle and same ammo it didn't do this.
- When I put the problem suppressor back on, it did it again.

So regardless of what a chart shows, empirical data is that suppressors are driving pressures in the rifle higher.

What brand had pressure and what brand didn't?

I believe I have similar symptoms with a nomad ti.

Isn't there a trend for "low back pressure" suppressors these days? Could this help?

 

[General RE LEE]

It's almost like the suppressor needs a minute to cool after a string and the back pressure to dissipate?

 

[Southern Lights]

What brand had pressure and what brand didn't?

I believe I have similar symptoms with a nomad ti.

Isn't there a trend for "low back pressure" suppressors these days? Could this help?

This is with the A-Tec H2 suppressor which is from Norway. I'm not sure they are imported into the US. The suppressor is fine on other rifles/ammo. But Hornady Superperformance was the culprit here.

I believe regardless of make, any suppressor can drive pressures up vs. bare muzzle. This is a common issue on semi-autos as well where running suppressed blew back much more pressure on my AR which is why adjustable gas blocks are often used.

 

[Southern Lights]

It's almost like the suppressor needs a minute to cool after a string and the back pressure to dissipate?

I was using on a 270 and was not rapid firing. The barrel and suppressor were cool when this was happening.

Basically, the suppressor is using baffles to slow down the gas expansion at the barrel. It's reasonable to assume this pressure may cause problems once the bullet leaves the barrel. The crowns get caked with carbon and the brass is dirtier when ejected as gas is likely coming back down the barrel.

Just know if you run suppressed that you may get pressure signs and they shouldn't just be ignored. The rifle is telling you something!

 

[Shortschaf]

I’d like to tag on this thread but I get this same issue with my Banish Backcountry. I get dirty chamber and pressure signs on occasion. This is with loads that are not remotely hot. It only happens on rounds later in a string. He first few shots on a cool bore no issue. Once the can starts getting hot, I get pressure signs primarily stiff bolt lift.

Have you ever cleaned the suppressor?
I've seen pressure (and dirty necks) go away to a large degree after cleaning a filthy can

 

[General RE LEE]

Have you ever cleaned the suppressor?
I've seen pressure (and dirty necks) go away to a large degree after cleaning a filthy can

I have not but I have noticed soot on some of my fired cases.