11.5 vs 10.5 Dwell Time?

**prepare** · 06-03-22, 06:22

Some years ago Paul Baffoni of BCM said that a 11.5 had almost 40% more dwell time than a 10.5

Is that statement accurate if both the 11.5 and the 10.5 have proper gas ports, buffers, and springs?

Not looking for over simplified information but at the same time not looking to make it overly complex either.

How do you go about determining the difference?

**odugrad** · 06-03-22, 06:33

What he said is correct. Dwell time doesn't have anything to do with buffers, springs, gas ports, etc.

Dwell time just measures the distance the bullet travels between the gas port and muzzle.

**Clint** · 06-03-22, 07:27

We like to call it Power Time, but Dwell Time is simply a function of barrel length, gas system length and muzzle velocity.

There is also the Blowdown Time, after the bullet exits the barrel, that adds to the gas drive.

BTW, blowdown is the phase that suppressors affect, as they slow down the blow down.

The overall operation is affected by gas port size, buffer weight and spring energy.

**Eurodriver** · 06-03-22, 07:37

Originally Posted by Clint

We like to call it Power Time, but Dwell Time is simply a function of barrel length, gas system length and muzzle velocity.

There is also the Blowdown Time, after the bullet exits the barrel, that adds to the gas drive.

BTW, blowdown is the phase that suppressors affect, as they slow down the blow down.

The overall operation is affected by gas port size, buffer weight and spring energy.

Tell me about this

https://blackrivertactical.com/WP/BR...UPP-p154960008

Do you have links to these in threads where they're discussed? Does it reduce gas in your face or just reduce the excess recoil associated with a suppressor?

**titsonritz** · 06-03-22, 13:10

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.

**Clint** · 06-03-22, 15:16

They're just like having a new smaller gas port and can be used for a variety of applications.

They only affect port size and cannot affect dwell time.

https://www.m4carbine.net/showthread...TUNE-Gas-Tubes

Originally Posted by Eurodriver

Tell me about this

https://blackrivertactical.com/WP/BR...UPP-p154960008

Do you have links to these in threads where they're discussed? Does it reduce gas in your face or just reduce the excess recoil associated with a suppressor?

**17K** · 06-03-22, 19:03

I agree that an 11.5” is a better mousetrap than a 10.3/5”, but only when they’re ported at .069”.

.076” takes all the advantage away IMO.

**Sean_gt** · 06-03-22, 21:17

This thread made me go look for that again. I tried to find what I thought I remembered as video a few weeks ago with no luck.

Tonight I found this post on TOS. I hope it's OK to paste here. If not please delete and google can take people there.

Q: Why BCM chose the 11.5" SBR over the 10.5"

I get this question a lot. It is a good one. When we spec our program we can build anything we would like, so we sat down and looked at the pros and cons.

First Statement: I know a good 10.5" SBR can run very well. I personally own a semi MK18 type and it runs great. No problems.

A: Dwell time.
Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle. When you pull the trigger and fire the weapon the movement of the bolt carrier group unlocks the bolt, extracts, and ejects the spent casing. Then it cocks the weapon, feeds, chambers the next round, and then locks again. One of the thing that can make SBRs finicky is the dwell time (or lack of).

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. IMHO, this is an excellent trade off. This additional dwell time (all other things being equal) will allow the carbine to be more forgiving to different ammo types, extremes in air temperature, weak or worn extractor springs, worn extractors, buffer weights, etc.

Last Statement: For those folks who have a 10.5" that works very well, I would reply; me too. (see first statement)

If I had to "go to war" with a AR15 Carbine, I would grab the BCM 11.5".
The BCM 11.5" Runs Like a Dream.

Hope this info helps,
Paul