How Does The Bolt Unlock?

[lysander]

I think we are talking about different things.
I am saying that the gas inside the gas tube prior to the pressurization (before and between shots) is much lower than 2500K, thus the speed at which the pressure pulse moves down the tube to the expansion chamber is way below 5,000 f/s. This is in agreement with and supportive of:

The gas tube of a carbine is 9.75 inches long if you straighten it out, so the gas has to travel about 10 inches to get from the gas port to the carrier cavity.

Looking at our graph in post #12 it takes a little less than 1/8 millisecond for the pressure appear at the cavity, so that means our gas has traveled a little less than a foot in a little less than 0.125 ms.

V(avg) = distance / time = a little less than (10/12ft) / a little less than (0.000125 sec) = a little less than 6,666 f/s

That's in the ballpark . . .

But, then there is the experimental data that shows:



. . . the peak velocity of the gas is around 1,500 m/s (4,921 fps, the red colored bit).

The speed in a carbine will be pretty much the same as the flow is choked to the speed of sound in the gas, despite the carbine having a slightly higher port pressure.

[Failure2Stop]

The gas tube of a carbine is 9.75 inches long if you straighten it out, so the gas has to travel about 10 inches to get from the gas port to the carrier cavity.

Looking at our graph in post #12 it takes a little less than 1/8 millisecond for the pressure appear at the cavity, so that means our gas has traveled a little less than a foot in a little less than 0.125 ms.

V(avg) = distance / time = a little less than (10/12ft) / a little less than (0.000125 sec) = a little less than 6,666 f/s

That's in the ballpark . . .

But, then there is the experimental data that shows:

https://i.imgur.com/5wPXAtj.png

. . . the peak velocity of the gas is around 1,500 m/s (4,921 fps, the red colored bit).

The speed in a carbine will be pretty much the same as the flow is choked to the speed of sound in the gas, despite the carbine having a slightly higher port pressure.

What is your data source for the pressure/timing breakdowns?
Is that model from Ansys Fluent?

I am asking because there are discrepancies between your provided data and my collected data, and I want to find the source data for investigation.
There are also issues with a lot of fluid modeling in firearms. Don't know why, but a lot of it simply does not line up with test results.

I'd like to get into more detail on this, but really all I can refer to with high confidence is our in-house testing and modeling, and putting work on my engineers for an internet discussion does not brief well to higher. Getting the source data and detail will reduce the task time to a reasonable side-conversation level of effort.

[lysander]

What is your data source for the pressure/timing breakdowns?
Is that model from Ansys Fluent?

I am asking because there are discrepancies between your provided data and my collected data, and I want to find the source data for investigation.
There are also issues with a lot of fluid modeling in firearms. Don't know why, but a lot of it simply does not line up with test results.

I'd like to get into more detail on this, but really all I can refer to with high confidence is our in-house testing and modeling, and putting work on my engineers for an internet discussion does not brief well to higher. Getting the source data and detail will reduce the task time to a reasonable side-conversation level of effort.

Yes, the graphs were generated by FLUENT. The data presented is from basically four reports:

1) "The Gas Flow in Gas Operated Weapons," BRL, 1970.
2) "Sensitivity Study of Rifle gas Systems", BRL, 1971
3) "Comparison of a Theoretical and Experimental Study of the Gas system in the M16A1 Rifle," BRL, 1971.
4) "Numerical Modeling of the Gas Flow in the M4 and M16 Rifle System.", ARDEC, 2009.

[Spooky1]

I would like to say thank you to y'all for sharing the information you guys are sharing.

[17K]

I would like to say thank you to y'all for sharing the information you guys are sharing.

This. This is one of the coolest threads on the ‘net.

[prepare]

The average temperature of the gas inside the gas tube is around 2,500 K, which gives a local speed of sound in the 5,000 fps range.

[bold] At this point in the cycle, the spring's contribution is negligible. Consider the spring pre-load is, at most 10 pounds, but the gas force is around 500 pounds. According to the model, if you remove the spring entirely, the bolt does not unlock any sooner.


Yet, Arfcom is full of people that can't seem to get their ARs to function properly . . .

Can you explain the temperature inside the gas tube? What is 2,500k? Do you mean degrees?

[Clint]

Degrees on the Kelvin scale...

Can you explain the temperature inside the gas tube? What is 2,500k? Do you mean degrees?

[JediGuy]

Degrees on the Kelvin scale...

I knew that one! But was way off on my estimated conversion now that I checked. High school was too long ago.

[prepare]

So what does 2500k translate to in Fahrenheit?

[georgeib]

So what does 2500k translate to in Fahrenheit?

Right at 4000 F.