View Full Version : Barrel length-Velocity-Pressure & Suppressors
I found this article and thought people might be interested. Here is the Link.
http://sadefensejournal.com/wp/?p=1093
Too bad that while they had a 16" barrel they didn't
Measure pressure at the carbine-length and mid-length gas system port locations.
Very interesting technique. But it doesn't really tell us anything we didn't know.
The most interesting part was the curiosity measurement at 3 inches from the bolt face. I did not expect to see 57,000 psi there with standard mil-spec ammo. I was aware the 5.56 chamber was rated for higher pressure than .223's 55,000psi, but I thought it was only 56-58,000psi. But according to Wikipedia the max pressure for 5.56 is 62,300.
It's amazing how much pressure is being contained a few inches from your face in these things.
SAAMI and NATO pressure testing methods are not the same and use different sensors. The authors of the article use a third method. All three give useful information but as they are different, the results cannot be directly compared
the results cannot be directly compared
maybe, maybe not. At the very least the trends should be comparable.
Too bad that while they had a 16" barrel they didn't
Measure pressure at the carbine-length and mid-length gas system port locations.
They basically did if you look at the curve. you just have to know the location of the gas port fior each gas system length down the barrel (7" vs 9" IIRC) and you can see the pressure at that location.
Barrel length should have no influence on gas port presure. That's just a function of where the port is located, and remaining barrel length should have no influence.
Barrel length should have no influence on gas port presure. That's just a function of where the port is located, and remaining barrel length should have no influence.
So pressure at the mid-length location for a 14.5" is the same as a 16"? or a 12.5"?
I'm a "show me" kind of guy and would much rather see the numbers than just have someone tell me that it's so.
So pressure at the mid-length location for a 14.5" is the same as a 16"? or a 12.5"?
I'm a "show me" kind of guy and would much rather see the numbers than just have someone tell me that it's so.
Pressure at a given port location is the same, regardless of barrel length. I believe the basic physics formula is P1V2=P2V1, What this says is that the pressure and volume have an inverse relationship. The volume we're talking about is the sum of the chamber volume and the barrel volume up to/just beyond the port and up to the base of the bullet -- basically what's pressurized is the volume behind the bullet; the volume ahead of the bullet isn't pressurized, so how much barrel extends beyond the port is irrelevant. So a gas port at the 7" point is going to result in the same volume between the bullets base and the chamber, regardless of a 12.5" barrel, 14.5", 16" 20" or whatever. The curve in the graph (Bore Pressure at Bullet Exit), just pretend your barrel length is 7" or 9" or whatever, that's what your port pressure will be. This graph allows you to interpolate these pressures at various port positions down the barrel.
What will change with barrel length is muzzle pressure. Muzzle pressure does not equal port pressure. With added barrel length, the volume gets larger so the resultant pressure decreases. Probably important if you are running a can, that goes to the article about running cans on SBRs, they have to endure much higher pressure than what a 16" barrel imparts to a can.
If all of this is true, I think the linked article is kind of pointless then.
If everyone already knows that pressure is higher at 4" than at 20", what difference does the article make? And what difference does barrel length make? and why did they chop the barrel down to do it? If what you way is accurate, at most all they needed to do was keep moving the sensor back, not chop the barrel.
and even if a 7" barrel has pressure of X 1" shy of the muzzle, the 20" barrel has the same pressure at that location?
If all of this is true, I think the linked article is kind of pointless then.
If everyone already knows that pressure is higher at 4" than at 20", what difference does the article make? And what difference does barrel length make? and why did they chop the barrel down to do it? If what you way is accurate, at most all they needed to do was keep moving the sensor back, not chop the barrel.
And even if a 7" barrel has pressure of X 1" shy of the muzzle, the 20" barrel has the same pressure at that location?
I think the takeaway is short barrels are hard as hell on cans due to the higher muzzle pressures resulting from shorter barrels and the higher temps at the muzzle.
Regarding your last question, no they are not the same. Pressures at the 19" point are far lower than pressures at the 6" point. This goes back to the larger volume of the 19" pressure vessel results in lower pressure than the trying to contain the same gas volume in a much smaller 6" pressure vessel.
I don't get your last point. Pressures at the 19" point are far lower than pressures at the 6" point. Where are you reading they're the same?
Let me rephrase
And even if a 7" barrel has pressure of X 1" shy of the muzzle, the 20" barrel has the same pressure at 6" from the bolt face?
Let me rephrase
And even if a 7" barrel has pressure of X 1" shy of the muzzle, the 20" barrel has the same pressure at 6" from the bolt face?
Re-read my post. Sorry, I read your last question too quickly and I thought it was a point you were making rather than a question. I edited my response before your last post.
If I'm reading your question above correctly, the answer is yes. the pressure vessel is the volume from the empty cartridge to the base of the bullet, wherever that bullet is in the bore. If it's travelled 6" down the bore, it doesn't matter if there's 1" remaining of dwell or 14" of remaining dwell -- the pressures contained within thase two vessels of identical volume are the same.
Pressure at a given port location is the same, regardless of barrel length. I believe the basic physics formula is P1V2=P2V1, What this says is that the pressure and volume have an inverse relationship.
To be an inverse relationship, your equation is wrong. Take PV = nRT. Your equation
should read P1/V2 = P2/V1, which gives P1V1 = P2V2. The port could be located anywhere
along the barrel and the result will be the same.
What will change with barrel length is muzzle pressure. Muzzle pressure does not equal port pressure. With added barrel length, the volume gets larger so the resultant pressure decreases. Probably important if you are running a can, that goes to the article about running cans on SBRs, they have to endure much higher pressure than what a 16" barrel imparts to a can.
Only if the barrel is so long that the temperature of the gas has started to decrease.
Temperature is proportional to Pressure and Volume. Volume can increase, but if temp.
increases as well, then pressure will be constant or increase.
Only if the barrel is so long that the temperature of the gas has started to decrease.
Temperature is proportional to Pressure and Volume. Volume can increase, but if temp. increases as well, then pressure will be constant or increase.
OK, but since temp doesn't increase this is a moot point. As every graph I've ever seen, and this one bears it out too, is that as barrel length increases, pressure within the vessel decreases. This is shown by the reduced muzzle pressures noted as barrel length increase.
I don't know for sure, but I would suspect that max temp occurs closely to max pressure which is where the powder is consumed and occurs within fractions, or maybe an inch or two after release from the case mouth. From that point fwd, pressure decreases as the bullet advances down the bore.
Jmart
Yes, you are right about the temperature aspect. Powder burns at a rate significantly faster than the bullet will travel.
I found this same test, in an abbreviated PDF;
http://www.dtic.mil/ndia/2010armament/WednesdayCumberlandPhilipDater.pdf
Does take some time to load.
Too bad that while they had a 16" barrel they didn't
Measure pressure at the carbine-length and mid-length gas system port locations.
So pressure at the mid-length location for a 14.5" is the same as a 16"? or a 12.5"?
I'm a "show me" kind of guy and would much rather see the numbers than just have someone tell me that it's so.
One point of clarification on the "port pressure".
At any given instant in time, The pressure behind the bullet is the same at all points along the barrel.
That pressure rises and falls with time, creating a pressure curve.
So the pressure at the port is simply a segment of this falling curve.
The segment starts at the time when the bullet passes the gas port and ends when the bullet clears the muzzle.
To answer your question, the 14.5" and 16" mid start at the same pressure and point on the curve.
They both follow the same curve, but the longer barrel rides it a little further out.
The longer barrel has more area under the curve, specifically at the end.
In order to maintain the same amount of gas transfer through the gas port, the longer barrel should slow down the rate of gas transfer slightly by using a slightly smaller port.
By the way, suppressors have same effect of extending the time of the pressure curve, NOT increasing actual pressure.
maybe, maybe not. At the very least the trends should be comparable.
You are correct. Let me clarify that I made my comments because Seth247 being surprised at a lower than expected pressure three inches from the chamber for NATO spec ammo and the common misconception that NATO ammo is loaded to higher pressure than SAAMI spec 223 ammo
If all of this is true, I think the linked article is kind of pointless then.
If everyone already knows that pressure is higher at 4" than at 20", what difference does the article make? And what difference does barrel length make? and why did they chop the barrel down to do it? If what you way is accurate, at most all they needed to do was keep moving the sensor back, not chop the barrel.
and even if a 7" barrel has pressure of X 1" shy of the muzzle, the 20" barrel has the same pressure at that location?
They were also testing the effect barrel length had on velocity and loudness of muzzle report
I saw these guy's at SHOT Show a couple months ago. I'm no engineer but what he showed me seemed to make sense as far as pressure goes. One of the things this suppressor design attacks is back pressure.
Here is the link to the website. http://oss-online.com/BPR.html
Respectfully,
--KJ
I saw these guy's at SHOT Show a couple months ago. I'm no engineer but what he showed me seemed to make sense as far as pressure goes. One of the things this suppressor design attacks is back pressure.
Here is the link to the website. http://oss-online.com/BPR.html
Respectfully,
--KJ
That site on the first page lists:
BPR™
MAD™
DRM™
PCD™
CTU™
HMS™
Yea, something tells me the marketing speak fail isn't just annoying to read, those likely aren't actual registered trademarks.
Just based on the silliness alone, I'd avoid that suppressor mfg. That said, AAC has said their newest silencers have changes to ensure less backpressure than their current M42k/spr cans, so there is certainly something to that.
Dater knows more about suppression than anyone alive most likely. This group of guys from Small Arms Review, LMO, Dan Shae really have been around and thoroughly know their stuff.
SAR just went to quarterly publications, both SAR is available online now and is a great investment, you can go and get the Sullivan AR articles now, I copy and paste them into a doc on my pc for future preservation.
what i took away from the article is that they ascertained the empiracle value (the theoretical values were available from software simulations) for which you must design a suppressor to be capable of withstanding for various barrel legnths.
for the purpose of finding the upper bound of this desired value it would make sense for them to exclude the gas port and the other parts of the gas system. it was already mentioned that it can be inferred what the pressures are at the different gas ports by just looking at what their results are at those lengths.
the other takeaway is that in the opinion of the authors, sub 10" barrels suck.
i think overall the intro addresses the interest by saying the shorter your barrel is the fatter your can has to be. it appears the opinion of the authors is that cans need to be built safely and the most important value as far as safe can manufacturing goes, is the pressure value at the uncorking. once that data is attained you can have a sure lower bound on how light/fat/cheap you can go on the can's material and geometry.
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