Technical Question on AR-15: Dwell Time

[advocatusdiaboli]

This is a very technical question and some will not be interested but fact and fiction are hard to distinguish on the internet.

What is dwell time?

I have heard, variously, that dwell time is:

• the length of time the bullet passes the gas port until it exits the barrel
• The time span that gas pressure builds from the gas tube into the bolt carrier group in a gas impingement system (not quite the same as the first definition though close)

I have heard the time the bullet is between the port and barrel end is called "cork" time and not "dwell" time to distinguish the two. That said, it makes sense it is the time the bullet "dwells" in the barrel. But what is it really?

Why is the distinction important so that I ask this question?

Dwell time takes a slice of the pressure curve in the barrel and depending where the slice starts and ends (due to gas port position in the barrel) on the irregular-shaped pressure curve, even if the same length, makes a huge difference in the maximum pressure delivered to the gas impingement system.

If you change the gas port size or where the port start and barrel length ends for different barrel lengths, the definition really matters:
the bullet spends the same time in the barrel but the pressure maximum is higher, builds faster, and it takes longer to dissipate so dwell time based on pressure would be longer but dwell time based on bullet travel time would be the same.

How can gas port size change accidentally then you ask?

Alignment of the port on the gas black with the port on the barrel during assembly. Even if off a little it can have consequences in operation.

[vliberatore]

I had always heard that dwell time was the time from gas port to the bullet exiting the barrel.

[Dhena81]

I always thought dwell time was the time that the brass case has until the bolt assembly starts to remove the case from the chamber.

Things that can change dwell time are cam pin channel geometry, the level of gas, or the weight of the reciprocating mass. I could be wrong technically lol.

[Munny$hot]

Once a round is fired and the bullet passes the gas port dwell is the amount of time that passes before the bolt starts to unlock from the extension.

[AreWeFree]

Here's some info that may help you:

http://ar15barrels.com/prod/operation.shtml

[advocatusdiaboli]

Here's some info that may help you:

http://ar15barrels.com/prod/operation.shtml

Thanks. Saw that before and read it through. Randall, who I respect a lot, defines dwell time in bullet travel measures not pressure. His chart is a great tool in visualizing the impact of barrel length and gas port distance under standard loads and pressures to bolt carrier pressure.

The problem is not all agree with his definition of dwell time which is why I asked. I tend to believe him over other though. That has worked very well in the past.

[AreWeFree]

Thanks. Saw that before and read it through. Randall, who I respect a lot, defines dwell time in bullet travel measures not pressure. His chart is a great tool in visualizing the impact of barrel length and gas port distance under standard loads and pressures to bolt carrier pressure.

The problem is not all agree with his definition of dwell time which is why I asked. I tend to believe him over other though. That has worked very well in the past.

I'm confused, his chart shows a pressure curve with markings to designate gas port length to barrel length under average conditions...

Does that not answer your questions on pressure? Regarding your definition, dwell time is exactly that, the time gas pressure is able to act. This occurs after the bullet passes the gas port and before it exits the muzzle.

To dwell is to linger, and this is a function of time, not energy.

[advocatusdiaboli]

I'm confused, his chart shows a pressure curve with markings to designate gas port length to barrel length under average conditions...Does that not answer your questions on pressure?

It already did answer previous questions on pressure (not in this thread) and it is pretty straight forward.The question was about the definition of the term "dwell time" and the implications people make using the term and actions they take to modify cycling behavior of ARs. Not his chart. I wanted to be clear on the term in detail.

I saw a video on YouTube about problems with SBRs cycling. In it the author stated that changing the gas port size changed dwell time. If bullet travel time or pressure time are dwell time, then that is not possible and the author would be incorrect. I wanted to be sure I could reconcile his view with Randall's chart.

What changes with gas port size is the amount of pressure in the system not how long (dwell time) it is there given we ignore any minor change in bullet velocity. With the increase in pressure in the system, the work it can (must) do in re-directing that pressure energy goes up. Not the dwell time at all in fact. Dwell time is a function of bullet velocity, barrel length, and gas port position but not size.

I'll believe Randall over some YouTube author guy on the "interwebs" but I was trying to verify it and fully understand it. That's all. I am an engineer and I process things that way: I want to fully understand things and have my model accurate so if I make changes, and will give me predictable measured results.

[CMBrowning]

Good read with lots of info!

[ar15barrels]

I saw a video on YouTube about problems with SBRs cycling. In it the author stated that changing the gas port size changed dwell time. If bullet travel time or pressure time are dwell time, then that is not possible and the author would be incorrect. I wanted to be sure I could reconcile his view with Randall's chart.

What changes with gas port size is the amount of pressure in the system not how long (dwell time) it is there given we ignore any minor change in bullet velocity. With the increase in pressure in the system, the work it can (must) do in re-directing that pressure energy goes up. Not the dwell time at all in fact. Dwell time is a function of bullet velocity, barrel length, and gas port position but not size.

I called it dwell time because it made the concept easier to understand.

Gas port size changes FLOW.
More FLOW will allow more PRESSURE to build in the gas system BEFORE the barrel uncorks and wastes all the excess pressure (supply) out the muzzle.
The gas system NEVER reaches equilibrium with the barrel before the bullet exits so we tune the gas operation by how much gas we let pressurize the gas system before the bullet uncorks the barrel and takes away our ability to further pressurize the gas system.

It's all about timing of the system to get it to function enough without functioning too fast.

[Lex]

Randall,

You got PM.

[Carcassonne]

I thought dwell time was how long the slide/bolt stopped moving after reaching the end of its travel.

[advocatusdiaboli]

I called it dwell time because it made the concept easier to understand.

Gas port size changes FLOW.
More FLOW will allow more PRESSURE to build in the gas system BEFORE the barrel uncorks and wastes all the excess pressure (supply) out the muzzle.
The gas system NEVER reaches equilibrium with the barrel before the bullet exits so we tune the gas operation by how much gas we let pressurize the gas system before the bullet uncorks the barrel and takes away our ability to further pressurize the gas system.

It's all about timing of the system to get it to function enough without functioning too fast.

Got it. thanks for that clarification. You are one of the best sources for this information any of us can access so I always hold what you say in high regard. Simplification to spread the information more widely is no sin but in fact a good move. Sorry to have bugged you about the details.

As an engineer, I just have to reconcile what I am told about firearms physics with my knowledge of physics or it bugs me and never stops. Yeah, it's a disease and addiction I know, but I cope and live a relatively normal life despite it (my wife would contest that statement though).