What Makes A Suppressor "Rated" For A Caliber?

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Specialized
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What Makes A Suppressor "Rated" For A Caliber?

Post by Specialized » Thu Sep 28, 2017 4:01 pm

Hi - New to these forums and I could use some edumacation. I have an Omega 9k on order for my CZ Scorpion, and since purchasing it I've been looking through the specs and such to see why or if this can, which is rated for 300BLK subsonic and supersonic with a minimum 10" barrel length, might be compatible with 6.8SPC with a 16" barrel as well. My reasoning is this:

- Both have SAAMI maximum pressures of 55,000psi
- Both rounds show muzzle velocities at nearly identical speeds
- Both shoot rounds that have similar weights

In asking Silencerco about it, they say that I shouldn't shoot 6.8SPC through an Omega 9k because it isn't "rated" for it, and when pressed, because the "gas volume and muzzle pressures of the 6.8 are too much" for it. So, my questions are this:

1. If these rounds shoot similar-weight bullets at a maximum of 55,000PSI each, what's the difference?
2. If both of these rounds have near-identical muzzle velocities, what's the difference?
3. If indeed there are "gas volume" and "muzzle pressure" issues, can someone please explain how these concepts work and how those differences would manifest themselves?
Please note, I'm not trying to be a smarta$$ here, my aim is to learn more about the fine art of understanding suppressors and how they work. It would appear to me that other than 6.8 being a smaller diameter than .308, the difference here would be minimal and both cartridges should be viable for use with an Omega 9k, but it's also quite possible that I'm overlooking some physics that actually are important to the equation. And I don't want to blow up a toy I've had to wait a year for because Congress can't agree on a lunch order, much less a simple bill. Any help you could provide would be much appreciated.

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John A.
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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by John A. » Thu Sep 28, 2017 5:02 pm

"rated" means it will withstand everything that the particular designation will withstand for its' service life. Not necessarily pressure alone.

Each caliber has a saami max pressures that are allowed. While saami concentrates on chamber pressure, that too also limits the pressures coming out of the other end.

So, "rated" means don't be going around trying to shoot something too powerful or too large through it. Otherwise bad things will happen.

Try shooting a 308 for extended time through a 9mm silencer.

While the 308 will fit through the bore just fine, it won't take long before you can is scrap metal.

Reference "gas pressure" and "erosion" and "metal and heat fatigue".
I don't care what your chart says

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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by Fulmen » Thu Sep 28, 2017 5:13 pm

Muzzle pressure has little to do with maximum chamber pressure. It depends mainly on the amount of powder and the total volume of the chamber/barrel.

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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by Specialized » Thu Sep 28, 2017 5:35 pm

John A. wrote:"rated" means it will withstand everything that the particular designation will withstand for its' service life. Not necessarily pressure alone.

Each caliber has a saami max pressures that are allowed. While saami concentrates on chamber pressure, that too also limits the pressures coming out of the other end.

So, "rated" means don't be going around trying to shoot something too powerful or too large through it. Otherwise bad things will happen.

Try shooting a 308 for extended time through a 9mm silencer.

While the 308 will fit through the bore just fine, it won't take long before you can is scrap metal.

Reference "gas pressure" and "erosion" and "metal and heat fatigue".
And this goes straight to my question: if chamber pressures and muzzle velocity are basically the same, and the projectile is smaller in diameter, then wouldn't one be able to safely extrapolate that both rounds are safe for a particular suppressor? I get your analogy of shooting a 308 through a 9mm can, where pressures are many times above what the can was built for, but unless I've forgotten a lot of my college physics, that analogy doesn't hold for shooting 6.8SPC through a 300BLK-rated can. Unless I'm missing something?

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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by Fulmen » Thu Sep 28, 2017 6:28 pm

Specialized wrote:if chamber pressures and muzzle velocity are basically the same, and the projectile is smaller in diameter, then wouldn't one be able to safely extrapolate that both rounds are safe for a particular suppressor?
Not really. A 300 delivers similar speeds & weights with less powder. The bore is also larger, providing more volume. So you should expect noticeably lower muzzle pressure from the 300. And even if the muzzle pressure were the same the larger powder charge will produce higher pressures inside the can.

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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by John A. » Thu Sep 28, 2017 6:37 pm

Fulmen wrote:Muzzle pressure has little to do with maximum chamber pressure. It depends mainly on the amount of powder and the total volume of the chamber/barrel.
I know.

But my point about chamber pressure I was trying to make, with Saami limiting chamber pressures to a certain amount, is safe to say that muzzle pressures are going to be a known given too.

You are also correct about taking less powder to create near the same bullet velocity.
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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by Fulmen » Fri Sep 29, 2017 1:52 am

Not sure I follow...
SAAMI (or CIP for that matter) doesn't regulate muzzle pressure in any way. It will of course be lower than peak pressures, but they're not related in any way.

A simple way to compare two rounds would be to divide the powder charge by total volume (a rough estimate valid for normal length rifle rounds would be caliber squared multiplied with the barrel length).
Example:
300BLK, 16": 16grs/(.308*.308*16)= 10,5
6.8SPC, 16": 28grs/(.277*.277*16)= 22,8

In other words you can expect roughly twice the muzzle pressure from a 6.8 using similar barrel lengths.

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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by John A. » Fri Sep 29, 2017 9:40 am

OK, I'll try to explain the point I was trying to make because I don't think you're following me.

Saami limits the chamber pressure. This much I think we both agree.

If the chamber pressure is limited to "X" amount, that means the pressure leaving the barrel is going to also have a maximum amount, whatever that will be.

I'm not saying that different barrel lengths and stuff can't change that amount of muzzle pressure because we know it will affect it, but the Mfg's should be able to make the determination of where they are comfortable of saying "we rate this can for..."

I'm not saying that chamber pressure and muzzle pressure are the same (we know muzzle pressure is going to be less), though when I am choosing a material to use, by default I normally do go by what the chamber pressure is because I know that the exit pressure isn't going to be as great so the material I choose should be able to withstand pressures greater than it should ever see. I would never choose a material that should handle the exact pressure it's going to handle or less. I always "over build" everything.

But I am saying that if you limit how much pressure you are going to have in the chamber (saami), the other is going to be affected proportionately. And while saami doesn't regulate muzzle pressure, in a way, it does that too because the muzzle pressure should never be greater than what's allowed in the chamber.
I don't care what your chart says

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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by Fulmen » Fri Sep 29, 2017 11:17 am

John A. wrote:if you limit how much pressure you are going to have in the chamber (saami), the other is going to be affected proportionately
Not really. Here's two loads from QL, 300BLK/16" barrel:
1. 125 Sierra/19grs H110:
Chamber: 50.5ksi
Muzzle: 4.8ksi

2. 240 Sierra/5.4grs HS6:
Chamber: 55ksi
Muzzle: 1.0ksi

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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by John A. » Fri Sep 29, 2017 2:01 pm

Again, I'm not disagreeing that different powders and bullets and subs and supers are going to behave differently, my point is the exit pressure is not going to be more than the chamber pressure.

By saami standards, there is a cap where chamber pressures are concerned so exit pressures are also going to be at or below those numbers by default.
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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by Specialized » Fri Sep 29, 2017 2:46 pm

Fulmen wrote:Not sure I follow...
SAAMI (or CIP for that matter) doesn't regulate muzzle pressure in any way. It will of course be lower than peak pressures, but they're not related in any way.

A simple way to compare two rounds would be to divide the powder charge by total volume (a rough estimate valid for normal length rifle rounds would be caliber squared multiplied with the barrel length).
Example:
300BLK, 16": 16grs/(.308*.308*16)= 10,5
6.8SPC, 16": 28grs/(.277*.277*16)= 22,8

In other words you can expect roughly twice the muzzle pressure from a 6.8 using similar barrel lengths.
Okay, I think this is what I've been looking for. So, for a HYPOTHETICAL ONLY, if Silencerco is making their Omega 9K's to withstand the pressure from a maximum muzzle volume of, say, 15ksi (a number made up purely in my head -- in no way associated with Silencerco), then 300BLK would fall within that spec, but 6.8SPC wouldn't, even though they have the same chamber pressure max of around 55ksi because the muzzle pressure is likely more than twice that for the 6.8SPC round than the 300BLK round. Did I state that correctly?

Another way to look at it would be that the case capacity has a direct influence on the barrel volume and, hence, muzzle pressure, just as the bore diameter does as well. At nearly double the case capacity and roughly 10% less barrel volume, if the formula above holds true, the muzzle pressure is way higher for the 6.8SPC. If that's true, then the issue is that the case pressure can be the same, but he muzzle pressure can be vastly different due to the smaller volume of the barrel from chamber to muzzle. Same reason a column of water moves faster through a smaller-diameter pipe than through a larger-diameter pipe.

The problem with this, of course, is that we are assuming the powder charges for each round burn at the same uniform rate. A slow-burning powder would generate less pressure than a fast-burning one, right? Not sure how to account for that, but it seems that burn rate could be factored in, if we knew what the max burn rate for all rounds in each caliber would be.

Isn't physics fun? LOL!

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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by Fulmen » Fri Sep 29, 2017 3:46 pm

John A. wrote:my point is the exit pressure is not going to be more than the chamber pressure
That is correct, but not very useful. Muzzle pressures for normal rifle rounds will generally be around 10-30% of chamber pressure, so the chamber pressure isn't very relevant when designing a silencer. Muzzle pressure alone isn't all that relevant either, the 223 and 308 both have similar chamber and muzzle pressures.

Specialized: You're catching on fast. But don't ignore the total amount of gas, it's just as important as the muzzle pressure. As for burn rate, it mainly affects chamber pressures. By the time the bullet leaves the muzzle all the powder have been consumed (more or less), so burn rate doesn't matter anymore.

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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by Specialized » Fri Sep 29, 2017 6:45 pm

Fulmen wrote: Specialized: You're catching on fast. But don't ignore the total amount of gas, it's just as important as the muzzle pressure. As for burn rate, it mainly affects chamber pressures. By the time the bullet leaves the muzzle all the powder have been consumed (more or less), so burn rate doesn't matter anymore.
Agreed. But again, I'm trying to understand what metric(s) are being measured that would rule out one caliber while judging another permissible through the suppressor. If you can't look at barrel volume as being representative (and I still think you can, given that it will tell you to some extent the velocity with which the gases will travel out the muzzle), then how does one measure the pressure that a round puts on the body of the can? In other words, how do I know when the round will turn my cylindrical suppressor into the shape of a bota bag? Obviously I want to avoid this, but what measurement tells me that the materials I'm using, if I were the engineers at Silencerco, are thick enough? Or, in my case, too thin (perhaps) to run 6.8SPC rounds through?

Maybe the question is: How do I compute gas volume based on the factors I know? I mean, the only way total volume is relevant beyond muzzle pressure is if the gas cannot escape the open hole in the suppressor fast enough to avoid its expansion of the body of the suppressor. This goes for the baffle design as well; if the baffles are constructed such that the movement of the gases are impeded by the baffles, more pressure will build up at the connection end of the suppressor than the open end. How do suppressor makers compute these values? Or is it trial and error?

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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by Fulmen » Sat Sep 30, 2017 8:01 am

Calculating actual stresses in a silencer is exceedingly hard. Predicting muzzle pressures is simple with software like Quickload, but once gas enters the can it becomes very complicated. I would assume most mfgs use a combination of CFD, FEA, experience and testing to ensure integrity. For the end user the safest thing is to stay within the muzzle pressure and powder charge of approved rounds. You can probably get away with a larger powder charge if the muzzle pressure is proportionally lower, but that's a WAG.

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Re: What Makes A Suppressor "Rated" For A Caliber?

Post by rentprop1 » Mon Oct 09, 2017 12:26 pm

I know that you can refer to each mgf's website to find these ratings, but anyone know if there is a List or comparison anywhere on the net ??

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