Dwell Time Percentage Differences

[prepare]

Dwell Time is amount of time the bullet spends in the barrel after it has passed the gas port.

Post Port Lengths (Carbine Gas System)

16” barrel = 9” post port
14.5” barrel = 7.5” post port
12.5” barrel = 5.5” post port
11.5” barrel = 4.5” post port
10.5” barrel = 3.5” post port


So what are the dwell time percentage increases and decreases between these barrel lengths?

[prepare]

For clarification Paul Buffoni at BCM said;
"The 11.5" carbine is approximately 4% longer weapon than its' 10.5" counterpart, but this extra inch gives the barrel a 40% increase in length for dwell time."

So what % longer is 12.5 than 10.5 and what % increase is the dwell time?

I'm not sure how to do the math...

[lonestardiver]

A lot deals with the velocity of the bullet from the gas port towards the muzzle and if it is still accelerating and if so what is the rate of acceleration. Then, is the acceleration at a constant rate or is it changing?

[AndyLate]

Look at the chart on the bottom of this page: http://www.ar15barrels.com/prod/operation.shtml

It does not show 12.5" numbers, but the dwell time should be roughly .140 - .150 ms.

Andy

[Clint]

As a rough approximation of time, you can just use the physical length at a constant (muzzle) velocity.

Of course, the bullet is always accelerating and velocity increases with barrel length.

What's important to remember is there is no universally ideal dwell time for all configurations.

Everything should be proportional when it comes to gas system parameters.

[1168]

That chart shows 10.5” and 11.5”, so adding the delta should get you reasonably close to 12.5”.

[MistWolf]

Dwell Time is amount of time the bullet spends in the barrel after it has passed the gas port.

Post Port Lengths (Carbine Gas System)

16” barrel = 9” post port
14.5” barrel = 7.5” post port
12.5” barrel = 5.5” post port
11.5” barrel = 4.5” post port
10.5” barrel = 3.5” post port


So what are the dwell time percentage increases and decreases between these barrel lengths?

Dwell time as defined above has nothing to do with function.

-If it did, the carrier would be pressurized and moving before the bullet left the barrel.

-If the carrier was pressurized and moving before the bullet left the barrel, it would be pressurized and moving while the bullet was in the same place in the barrel regardless of distance from port to muzzle.

-If the carrier was pressurized and moving when the bullet was in the same critical place in the barrel, adding barrel length after the gas port location would not increase gas flow to the carrier thus increasing carrier speed.

-If the carrier was pressurized and moving when the bullet was in that critical place in the barrel, adding a suppressor would not increase gas flow to the carrier thus increasing carrier speed.

However, adding barrel length after the port or adding a suppressor does increase flow to the carrier and does increase carrier speed. Therefore, something else must be going on.

In fact, the carrier doesn't pressurize and begin moving until after the bullet leaves the muzzle. The system is pressurized by the residual pressure left in the system after the bullet exits the muzzle. The longer the distance from the gas port to the muzzle, the longer it takes the system to blow down.

A suppressor has the same affect. Suppressor reduce noise by cooling the gas and slowing down its escape from the muzzle. This keeps pressure in the system higher for a longer period. This increases flow to the carrier and thus increases carrier speed.

The time difference for a bullet to travel an extra 1.5 inch is a fraction of a millisecond and has nothing to how the AR functions. However, blowdown rate is very important to function and span of operation.

[Rayrevolver]

Dwell Time is amount of time the bullet spends in the barrel after it has passed the gas port.

Post Port Lengths (Carbine Gas System)

16” barrel = 9” post port
14.5” barrel = 7.5” post port
12.5” barrel = 5.5” post port
11.5” barrel = 4.5” post port
10.5” barrel = 3.5” post port


So what are the dwell time percentage increases and decreases between these barrel lengths?

I did some of the math in this post:
https://www.m4carbine.net/showthread...07#post2173607

[prepare]

I did some of the math in this post:
https://www.m4carbine.net/showthread...07#post2173607

Thanks for link to your calculations.