Barrel length-Velocity-Pressure & Suppressors

[wideglide]

I found this article and thought people might be interested. Here is the Link.
http://sadefensejournal.com/wp/?p=1093

[rob_s]

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.

[Seth247]

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.

[MistWolf]

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

[rob_s]

the results cannot be directly compared

maybe, maybe not. At the very least the trends should be comparable.

[jmart]

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.

[rob_s]

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.

[jmart]

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.

[rob_s]

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?

[jmart]

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.