11.5 vs 10.5 Dwell Time?

[Todd.K]

If it cycles a bit faster than a different barrel it’s more likely to drop a primer.

[Sparky5019]

If it cycles a bit faster than a different barrel it’s more likely to drop a primer.

It doesn’t. A simple barrel switch fixed the issue. It was in the barrel one way or another.


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[Todd.K]

What was the cyclic rate before and after the barrel change?

[Sparky5019]

What was the cyclic rate before and after the barrel change?

I didn’t check it. The only time I check true cyclic rates is when timing auto sears. It shot and functioned exactly the same though with only a barrel change.


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[prepare]

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

I'm have a hard time mathematically figuring out where the 40% comes from?

[berkeleygiraffe]

I have no evidence of this, but I have the feeling the 11.5 thing came about because BCM is a manufacturer and makes a lot of rifles, and as a manufacturer they have to warranty repair and diagnose rifles that people complain aren't working right or aren't working at all. With all of the people out there that they sell to, some will use underpowered steel 223 while others use nice hot 556. if they open up the port on a 10.x barrel then the 223 cycles fine but the 556 kicks harder and wears out parts faster. if the keep the port optimized for brass 556 suppressed, then 223 won't cycle. it's harder to please both parties with one barrel port size.

the "dwell time" is the time the bullet is past the port and keeps pressure "on" before the bullet exists and releases the pressure. with a 11.5, there's a longer period after the bullet passes the gas port than on a 10.x barrel. that opens up forgiveness and allows manufacturers to see less complaints about their barrels from both the 223 steel crowd and the 556 suppressed crowd.

i really like what clint and brt are doing in that they are giving users the option to tailor the gas tube port size to the type of ammo and system the end user shoots.

[Pappabear]

It doesn't work that way.

Punching out the gas port size isn't the same as changing the gas system configuration.

Short dwell configurations are more sensitive to variables in ammo, environment and suppressor use.




Because the 11.5 works reliably over a wider range of variables than a 10.x. due to the better gas system configuration.


We deal with this on a daily basis working up solutions with the EZTUNE gas tubes.

The 11.5 can be setup pretty nicely to run 50/50 suppressed on a single port size.

The 10.5" doesn't do 50/50 nearly as well because the port size can hardly be reduced and maintain un-suppressed reliability.

Then when the suppressor is added, it's way more gassy than an 11.5" setup the same way.

We generally recommend a 10.5 be used as either un-suppressed only or dedicated suppressed, which works great with a small port.

That recommendation is based on having full control over gas port size, so port size isn't the issue, the gas system configuration and dwell time is.

The 10.5 can be reliable at one or the other, but not both with the same port size like the 11.5 or 12.5.

Clinster, that was a solid post, good on you. I have both 10.3's, 10.5's and 11.5's that run well and I have often wondered what the bottom line on the 10.3 10.5 vs 11.5 story was all about. Good explanation. It's one reason I run suppressed 100% of the time.

Euro, it takes the AR10 gas guns too.

PB

[prepare]

What kind of math formula did Paul Bafoni use to determine a 11.5 is 4% longer than a 10.5?

[1168]

What kind of math formula did Paul Bafoni use to determine a 11.5 is 4% longer than a 10.5?

Overall length of the gun, probably.

[MegademiC]

I'm have a hard time mathematically figuring out where the 40% comes from?

This was answered and akcnowledged by you already, or am i missing something?

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.