Does A Can or Suppresor Increase Back Pressure and Decrease Velocity??? Well????

[Stevenstl86]

As everyone has probably said already. Pending Suppressor length and design will increase velocity but usually not enough to change dope but may change poi. Chamber pressure just last a little longer but not increase.

 

[Veteran]

Can we really compare a gas gun ejection system changes because of adding a suppressor to a bolt gun chamber pressure? Seems like two totally different subjects to me.

Well having that semi auto gas return tube in the data proves there is back pressure being added to the system when a can is installed. We wouldnt have that piece of the data or the puzzle if all we had was bolt guns in the data.

So I think its valuable for that reason for sure.

 

[OldElkHunter1961]

WRT "vents" drilled into an existing barrel the length of the drilled/vented portion of the barrel effectively decreases barrel length by that distance (22" barrel, drilled segment 1.5" = 20.5" new effective barrel length). How much velocity is lost depends on what percentage of total barrel was "removed." The shorter the barrel the greater percentage of barrel lost. WRT suppressor effects on velocity I've only seen anecdotal changes in bullet speed. Meaning low single to low double digit increases. Only my experiences, I'm not a ballistics laboratory. Just a curious shooter. Enjoy the tinkering.

 

[coop2564]

Every gun I have used one on had slight increase in velocity. 8 to 15fps.

 

[vortech347]

Yes I think the gas gun example compares to any gun with a can. It's a direct example of increased back pressure that you can't measure in a bolt gun.

I have not tried to determine if a bolt gun gets dirtier shooting suppressed vs. unsuppressed.

But a DI gun like an AR gets WAY dirtier and real fast when shooting suppressed. And case ejection velocity goes way up.

 

[vortech347]

Yes, I think this has helped me understand the conflict between my mental model of backpressure which does seem to happen in the rifle system, and the conflict of increased velocity. One would think in a simple world of physics, back pressure would equal decreased velocity, but what is happening is precisely as you say, the increased effective confinement of some length of tube where gas is still behind the bullet exerting force overcomes the back pressure effect so the net effect is increase velocity.

So, its another example of counter intuitive results in the real world vs. a simplistic view of physics where a +b should = c.
But it don't.

Higher charge weights should always mean more velocity right? But the Satterlee curve says it don't. Different thread.
Same counterintuitive kinds of things going on.

Actually, I don't think it is counterintuitive. Look at AR's and where they put the gas port and size depending on barrel length. A rifle gas system has a smaller port and it's further down the barrel. That seems to support the idea that the longer barrel has more backpressure, too. If you put a carbine length gas port and size on a rifle length barrel you'll be way overgassed.

So the main reason a can adds velocity, even though it's bigger than the bore and unrifled, is the fact it effectively adds a very small amount of barrel length to the system.

Good thread.

 

[Stevenstl86]

Well having that semi auto gas return tube in the data proves there is back pressure being added to the system when a can is installed. We wouldnt have that piece of the data or the puzzle if all we had was bolt guns in the data.

So I think its valuable for that reason for sure.

Do you have a table that show back pressure gains? I'm really curious to see the difference in rounds and combos

 

[Mike Matteson]

I have only one rifle that has a brake on it. It was added to the end of the barrel and not cut back. I have another rifle coming (someday) with a barrel length 2" longer or 26" barrel plus brake. Both will or are going to use the same basic case and powder loads (to be determine in load make up). The second rifle being built will have 30" barrel length with a 7 twist rate in a 6mm/289AI case. Now I am having it built to accept a suppressor or a muzzle brake. Which will make an extremely long rifle, and I realize that too. The barrel is from Bartlein (Heavy Palma) 32" to be cut back to 30".
I fine this very interesting, because I had given it some though on how or what might be a problems with either type of use. I was wondering with the suppressor, what it could do or what problems it would create. In working up load for those two rifles I will have to see what happens. I will sure take it into account when building a load. Thanks for the insight. The other is I generally use H powders to reload with. (H435/H4831/H1000) and non mag primers.
What I haven't seen was suppressors like the Army had in Vietnam had. They were about 18" long and about 1-1/2 O.D. if I remember correctly.

 

[MKWas]

Yes, I believe you are right.
The muzzle ventilator in the book Rifle Accuracy Facts was cut/drilled into an
existing end of bbl, and the
velocities dropped I think because
gas escaping the bbl itself does not drive the bullet, but if the brake is an extension of the bbl, it would be different. I stand corrected.

That makes sense. I'd expect a suppressor or brake that extends the rifle to increase pressure & velocity slightly though not as much as a longer barrel would since some gas is vented.


Also, I believe that neither would increase peak pressures in any reasonable load & barrel..

Pressure increases as powder burns because it generates more gas (this effect is most pronounced when the powder burns at the maximum rate), and it decreases as the bullet travels down the barrel because the volume containing the gas increases. So by the time the bullet reaches the suppressor or brake the powder should have mostly burned and the instantaneous pressure is below the peak pressures. The brake or suppressor will slightly increase pressure when compared to the bare barrel.


By reasonable load I mean the powder is efficiently burned, mostly before the bullet reaches the end of the barrel. Peak burn depends on the powder but should occur while the bullet is still well within the barrel so it can impart energy to the bullet. A practical load requires trade-offs it will waste some powder, a powder that burns instantaneously will have huge pressure peaks.

I'm also assuming that the barrel ahead of the bullet does not affect the instantaneous internal ballistics. The only effect I can think of where the barrel ahead of the bullet has an effect is that the bullet must compress & push air out of the barrel. So extending the barrel in any way increases the amount of air that must be expelled, it will decrease velocity & increase pressure, but since air is so thin it should be negligible for a barrel of any reasonable length. If this weren't the case we'd be shooting shorter barrels.

 

[bengineer]

I haven't read all the responses on pages 2-4, but I am tired from reading the original post. Tired because there are several false assumptions and misunderstandings that I fear people walk away from this with gravely mistaken notions. I do not mean to condescend, but merely to inform. I don't correct people's grammar or spelling on this forum because it is condescending.

Pressure is measured in force per (unit) area. We always break this down to single unit area, rather than leave it as a weird fraction. Gauges display in unit area, whether you prefer psi or Pascals.

Pressure absolute (psia) is what we are under anywhere we are, based on what we are in. At the bottom of a swimming pool, we have the mass of the water and the air in the atmosphere pushing down on us. At the side of the same pool, we have just the air (unless the HVAC system isn't balanced). On the moon, we are at 0psia. The inside of your bore and the inside of your can attached to said bore are at the same pressure, whether you're atop Everest or in Death Valley. Please, let's not think of that assembly at the bottom of the swimming pool.

Pressure gauge (psig) is what is measured on a gauge, inside a pressure vessel, such as a basketball, or your barrel/silencer. A gauge that is not attached to anything, just in your hand, will read 0 if it is calibrated correctly. The gauge pressure may also read negative, indicating the gauge pressure is lower than the ambient pressure (inside a conventional light bulb, for example - though I have never seen a pressure gauge on a light bulb).

Backpressure. This is a word often thrown around, but is quite misleading. Pressure is pressure. Increased resistance can increase pressure in a system, such as your gun barrel (longer-than-expected bearing surface, rust, mud, etc). Decreased resistance, such as HBN or Moly on bullets, or cast bullets, can decrease pressure in a system. Adding a can/silencer to a barrel doesn't increase resistance, but it does increase dwell time of the bullet and expanding/expanded gases, which slows the decrease in pressure as compared to no can/silencer. This gives the illusion of "backpressure," but the can really doesn't create anything; the can just holds pressure longer. That provides the perceived recoil mitigation of the can (mostly). But it does not cause "backpressure."

Same goes for cans on a gas gun. More combustion gases come out of the gas port and into my right eye because pressure is held longer in the can. Also, the slower decline of pressure due to the can may also push more gas into the gas system, possibly sending even more gases into my shooting eye.

I have fiddled a little with coefficients of friction or surface polish of both reloading dies and chambers. Just a little, though. I haven't seen anything conclusive to lead me to see that brass needs roughness or chambers need some level of roughness/texture (as opposed to absolute mirror finish) in order for things to work correctly, or prevent case head separation, or primers from backing out. I have heard all manner of stories and opinions, from 'brass has to grip the rough chamber or it will cause brass flow out of the action' to 'chambers have to be smooth or the brass will grip and cause primers to back out without the case head pushed against the bolt face.' None of these kinds of statements are based in quantitative fact, but are of opinion. Much like some other areas of interest, say, on the TV at 6:00. Opinions are important to their respective owners, but they are not valuable in discussions like this thread. And don't forget that an anecdote isn't enough sample to have a valid conclusion.

 

[vortech347]

Do you have a table that show back pressure gains? I'm really curious to see the difference in rounds and combos

Not answering for OP, but I've not seen anyone in the past put out that info. What I can tell you is a AR shooting suppressed with no change to the gas system will eject cartridges much harder and further forward than non-suppressed. So the gas system is certainly seeing more pressure. It acts like an over-gassed rifle and you can tune it back to normal with an adjustable gas block.

 

[cohunt]

In a gas gun we can agree that the rifle exhibits signs of being "over gassed" when suppressed--- but is this actually a higher chamber pressire-- or just a longer pressure curve? We know that a can "holds the pressure in the barrel longer" but im not convinced its due to a higher chamber pressure.
gas guns operate on a "balanced gas system" --- you need a certain VOLUME of gas at a certain PRESSURE for a certain length of TIME ( iirc this is Boyles law)...

So what we see as an increase in pressure, could be an increase in time or volume too.....without measuring chamber pressure there would be no way to be sure---- I think its just an increase of the same pressure over a linger time that causes these "over gassed ssymptoms"

We do know that when you "un cork" the barrel that gas pressure dramatically drops.

My chart I'm posting doesn't relate to cans, but it does show different pressures at chamber, gas port, and muzzle I'm reference to barrel length

 

[vortech347]

In a gas gun we can agree that the rifle exhibits signs of being "over gassed" when suppressed--- but is this actually a higher chamber pressire-- or just a longer pressure curve? We know that a can "holds the pressure in the barrel longer" but im not convinced its due to a higher chamber pressure.
gas guns operate on a "balanced gas system" --- you need a certain VOLUME of gas at a certain PRESSURE for a certain length of TIME ( iirc this is Boyles law)...

So what we see as an increase in pressure, could be an increase in time or volume too.....without measuring chamber pressure there would be no way to be sure---- I think its just an increase of the same pressure over a linger time that causes these "over gassed ssymptoms"

We do know that when you "un cork" the barrel that gas pressure dramatically drops.

My chart I'm posting doesn't relate to cans, but it does show different pressures at chamber, gas port, and muzzle I'm reference to barrel length

I believe you are right. It's dwell time, not increased pressure. It's the same in cars. Backpressure is not really the right word to use but everyone is used to it. It's more about scavenging and removing restrictions to expanding gases than something pushing back.

 

[lancetkenyon]

So, I will clarify my set ups for reference.
Suppressors: TBAC Ultra 7 6.5mm, TBAC Ultra 9 .308", TBAC Dominus .308".
All use the TBAC CB on the rifle for mounting the suppressor.
I ONLY shoot these rifles suppressed now, and never shoot without the can I have the load worked up for off the rifle. I also do not switch cans once the load is worked up. But, all my cans do go back and forth between multiple rifles. I remove my cans at the end of the day, to store my rifles and cans in the safe.

24" 6 Creed w. Ultra 9
26" 25 SST w. Ultra 9
26" 25 Creed w. Ultra 7
22" 6.5 SLR w. Ultra 7
24" .260AI w. Dominus
20" 6.5SS w. Ultra 7
26" 6.5SS w. Ultra 7
26" .280AI w. Ultra 9
24" 7SS w. Dominus

I have thousands of rounds through my Ultra 9 and Ultra 7 between cleanings. The Dominus is only a few months old, so only has a couple hundred through it, and has not been cleaned yet. I notice a bit of carbon build up on the CB, but not alarming. When I cleaned the cans, I got a LOT of built-up carbon out. TBAC states their cans are almost "self-cleaning".

I shoot over a Labradar whenever I am at the range, and I never noticed a drop or increase in MV/ES/SD between a clean or dirty can. I think my highest ES in any of my final loads is around 12. Most are single digits. I record a lot of rounds for MV, so I am confident in my numbers. I can tell a bit of difference in the 1st round "pop" with a clean can.