11.5 vs 10.5 Dwell Time?

[prepare]

How is the 11.5 has 40% more dwell time than a 10.5 calculated?

[Joelski]

Just a guess, but gas to muzzle length must be 40% longer thanks to the added overall length.

[Todd.K]

The whole 10.3"/10.5" number came around because it is the shortest length barrel that can still use a standard FSB and carbine gas system, but they can be finicky. I won't go any shorter than 11.5" in 5.56 due to loss of velocity (terminal performance) and dwell time (operational performance). With a 11.5" vs 10.5" you get 4% increase in length of weapon with a 40% increase in dwell time, which translates to a very small increase in size vs a major increase in reliability.

I’m not saying this to pick on you or BCM, but it’s just good marketing. The hard truth is that dwell doesn’t matter much at carbine gas length and 40% dwell time makes most people think it runs 40% better. Dwell time is a real concern at the much lower port pressure of rifle gas length.

The problem with carbine gas is that it exposes things like lack of buffer weight, weak extractor springs, damaged or poor magazines, and crazy port sizes. All things that have been figured out and are fixable, but are known even on the 14.5”. The difference from 14.5” all the way down to 10.5” is just very, very marginal if the above things are taken care of.

[prepare]

So is there even any difference in the dwell time between an 11.5 vs a 10.5?

If there is a difference how is measured?

[Todd.K]

Dwell time is how long the gas port is under pressure. So from the gas port to the end of the barrel.

If the gas length stays the same, a longer barrel has more dwell. Both the 10.5” and 11.5” use carbine length gas systems, so the 11.5” has more dwell.

My position is that it doesn’t really matter, both can be reliable.

I think you should focus on getting a quality barrel that isn’t known for excessive port sizes first, the length difference second.

[prepare]

Dwell time is how long the gas port is under pressure. So from the gas port to the end of the barrel.

If the gas length stays the same, a longer barrel has more dwell. Both the 10.5” and 11.5” use carbine length gas systems, so the 11.5” has more dwell.

My position is that it doesn’t really matter, both can be reliable.

I think you should focus on getting a quality barrel that isn’t known for excessive port sizes first, the length difference second.

I'm not in the market for a barrel.

I'm trying to understand the statement Paul Bafonni made and how does one determine the percentage of dwell time the 11.5 has over the 10.5

[Clint]

Yes, there's obviously a difference between the two.

One way to calculate dwell time:

take the distance from gas port to muzzle and divide by average bullet velocity

The 10.5" has 2.83" and the 11.5" has 3.83"

If you use an average velocity of 2600 fps and 2675 fps (and convert to inches), you'll get approximately

91 microseconds for the 10.5"
119 microseconds for the 11.5"

That difference is 32% more by our math, and the difference between 10.3 and 11.5 is about 44%, so perhaps the 40% number is close enough to the average of those.

[prepare]

Yes, there's obviously a difference between the two.

One way to calculate dwell time:

take the distance from gas port to muzzle and divide by average bullet velocity

The 10.5" has 2.83" and the 11.5" has 3.83"

If you use an average velocity of 2600 fps and 2675 fps (and convert to inches), you'll get approximately

91 microseconds for the 10.5"
119 microseconds for the 11.5"

That difference is 32% more by our math, and the difference between 10.3 and 11.5 is about 44%, so perhaps the 40% number is close enough to the average of those.

Thank you for the calculation.

[Clint]

Keep in mind, one shouldn't focus on a single parameter without considering all the other related parameters.

There is no universally "correct" dwell time.

A better version of that question would be:

"Given a barrel of X length, Y cartridge chamber and Z suppressor use, where should the gas port be located for best performance?"

[prepare]

Keep in mind, one shouldn't focus on a single parameter without considering all the other related parameters.

There is no universally "correct" dwell time.

A better version of that question would be:

"Given a barrel of X length, Y cartridge chamber and Z suppressor use, where should the gas port be located for best performance?"

I was mostly trying to understand Paul Baffoni’s statement, if it’s accurate, and how he arrived at it…