11.5” Mid Length Barrel Discussion

[Rayrevolver]

This is a technical thread to specifically address the 11.5” mid length gas configuration, suppressed 24/7. Specifically this barrel from Faxon:
http://faxonfirearms.com/11-5-big-gu...ngth-4150-qpq/

While we wait for Faxon to get us the above configuration barrel, I will try to nerd out on the basic M-16 operating system, pontificate on technical solutions, and make bad assumptions.

THAT SAID I am hoping SMEs can come in and correct bad information/assumptions. And I will steal this nomenclature from Clint where 11.5M = 11.5 Mid Length and 11.5C is the carbine length gasser.

Disclaimer: I do not work in the firearms industry.

[Rayrevolver]

FIRST ASSUMPTION: The M-16 gas operating system is the baseline for all our comparisons and will be considered the ultimate in reliable function with milspec ammo.

Using the AR15Barrels.com data (table at the bottom of the link below), that means a 20” barrel, 13.2” gas length, M193 @ 3244 fps, and a dwell time of 0.184 ms.

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

11.5M, 12.5M and 12.5C are not in the data, so those are my own calculations (and % decrease from the baseline).

Barrel - Dwell Time - % of baseline dwell

Baseline (20" Rifle aka 20R)
20.0" 0.184ms

Mid Length Gas
11.5” 0.053ms 71% less dwell time (or 29% of the dwell time of the baseline)
12.5” 0.079ms 57% less dwell time
14.5” 0.139ms 25% less dwell time
16.0” 0.181ms 2% less dwell time

Carbine Gas
10.5” 0.090ms 51% less dwell time
11.5” 0.120ms 35% less dwell time
12.5” 0.149ms 19% less dwell time
14.5” 0.206ms 12% MORE dwell time
16.0” 0.248ms 35% MORE dwell time

It does look like the 16M dwell time is the same as the 20R baseline. ***EDIT*** New numbers for 12.5C is .149ms and 19% less dwell. That means it should be closer to the baseline than the 14.5M. Thanks Clint for catching that.

[dentron]

Per the item description :

Due to the short dwell of the barrel, the gas port will be larger than most barrels. Those looking to use suppressors should look to an adjustable gas block

[Rayrevolver]

Yes and I could have swore it used to also say the gas port was ~0.098.

This project is what got me looking into adjustable gas blocks. I wanted to go the Govnah route but this barrel has a .625 journal. The Baby Govnah is light, but also not a player. Looked at BRT Micropin but also .750 only.

I have a SLR Sentry .625.

[Gunfixr]

Interesting. I have my own thread concerning a very similar barrel. Although, I am leaning heavily towards 12" as the compromise.
I could go 11.5" already made, as I would otherwise be cutting down a mid 14.5", and recrowning.
Due to the specifics of my plan, a nitrided barrel will give problems during the build, so I was leaning toward a regular cl barrel.

Sent from my SGP612 using Tapatalk

[Clint]

Last I heard, that Gunner barrel was actually going to be carbine length gas instead of midlength.

We actually have the .625" version of the MicroPin Gas Block coming out next month.

[FaxonNathan]

Our intent was to create a lighter barrel that fits the minimum length that most suppressor companies warrant for full-auto fire and the best timing possible. Mid length fit the bill.

Barrels are done being profiled and are out to QPQ. Should not be too much longer!

Last I heard, that Gunner barrel was actually going to be carbine length gas instead of midlength.

We actually have the .625" version of the MicroPin Gas Block coming out next month.

We did the testing and 11.5" works like a charm. Now, the difference between the unsuppressed and suppressed is fun, but an adjustable gas blocks works well. We figured most users were going to basically leave the can attached full-time.

Per the item description :

Due to the short dwell of the barrel, the gas port will be larger than most barrels. Those looking to use suppressors should look to an adjustable gas block

Precisely. Gas port will be around ~.110"-.115"

[tom12.7]

11.5" 5.56 AR's are a study onto themselves in many ways. In the past when looking at a suppressed only version of these, it adds complications. The ways different cans altering function is another variable thrown in. To keep it simple though, a 11.5" 5.56 using one can type only may produce improved results when lengthening the gas system to simulate how it interacts as if it was a longer barrel. When tested with available ammo at the time, there is a range of carrier velocities that has a curve when comparing the delta in rates or increments of change. If those changes are graphed, there is a noticeable dip that occurs in the change in those rates for most. When you add multiple types of cans, the gas system length that reduces the changes in carrier speed changes does change longer or shorter in gas system length. When a variety of the available ammo was included, a consensus was that a +1" gas system improvement was a better choice overall over the +1.5" gas system for most.
As with anything YMMV, but from testing, I would be more comfortable for most with the +1" gas system over the mid gas on the 11.5" 5.56 suppressed only gun with multiple choices of cans and ammo, obviously a certain can and ammo may result into something different.

[dentron]

11.5" 5.56 AR's are a study onto themselves in many ways. In the past when looking at a suppressed only version of these, it adds complications. The ways different cans altering function is another variable thrown in. To keep it simple though, a 11.5" 5.56 using one can type only may produce improved results when lengthening the gas system to simulate how it interacts as if it was a longer barrel. When tested with available ammo at the time, there is a range of carrier velocities that has a curve when comparing the delta in rates or increments of change. If those changes are graphed, there is a noticeable dip that occurs in the change in those rates for most. When you add multiple types of cans, the gas system length that reduces the changes in carrier speed changes does change longer or shorter in gas system length. When a variety of the available ammo was included, a consensus was that a +1" gas system improvement was a better choice overall over the +1.5" gas system for most.
As with anything YMMV, but from testing, I would be more comfortable for most with the +1" gas system over the mid gas on the 11.5" 5.56 suppressed only gun with multiple choices of cans and ammo, obviously a certain can and ammo may result into something different.

Thank you for your input.

I have heard of these carbine +1" gas systems before but have never seen one or heard of anyone selling them. Is this custom only?

[Clint]

Ray,

Your analysis looks good so far. (You may want to double check your estimates for the 12.5" length. I have that at .150ms. )

There is far more involved than just dwell time.

There is also the blowdown time and pressures at the gas port and muzzle.

Together these form the overall pressure curve.

The total area under the curve ( pressure x time ) sets the available gas drive to cycle the weapon.

The gas port size regulates the flow and effectively scales down the total gas drive.

The gas tube length adds delay to the start of the cycle.

The total cycling mass ( buffer + BCG + 1/2 spring ) affects the acceleration of the carrier and affects the start of extraction.

The start of extraction relative to when the bullet exits the muzzle and pressure drops rapidly is also an important factor.