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.
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.
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.
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
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.
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.
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!
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.
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.
From what I’ve seen, I would be hesitant about a conventional mid length gas barrel in a 11.5" 5.56 suppressed only gun for generic purposes. There are too many variables, the stretch is too far in gas system length. Slightly shortening this to something similar to a 1" extended system does offer some gain. Oversizing the port with an adjustable gas block may produce acceptable results for some, but others may really want a dedicated port without user adjustments for them. If one used a non adjustable block, could their span of function be reduced? Some, with a limit of choices of ammunition and sound suppressor for use be a possible reduction or gain in span of operation?
If one does not use an adjustable gas block, their span will be the same. Recoil will increase over time (like all barrels as gas ports wear).
We’re not claiming (or marketing) this barrel to be for everyone. This is very much for advanced users who understand the AR in-depth and the benefits and trade-offs that come with this configuration.
Rut Row. Not an advanced user here, but I understand this is an experiment so to speak.
It is known that the shorter dwell time means less flexible with respect to ammo choices. To that end, if you wear belts and suspenders the 20R should be your SHTF weapon.
Technically anything else accepts more risk of not working with pickup ammo should that become a need. For me and reasonable scenarios, a 12.5C is my go to.
For this project, I have not decided what ammo to tune it for. My cheapest is Wolf Gold 55gr (Taiwanese brass cased).
The other wild card here is also the fact that heavier and slower ammo means more Dwell Time! If I cannot get the 11.5M reliable with 55gr ammo then I will try the 77gr Mk262 clone ammo.
My FIL has a plain vanilla 11.5C BCM with an FSP. I will function test the Faxon barrel unsuppressed and will try to shoot these uppers together.
In a compare and contrast scheme of looking at things like a suppressed only 11.5" 5.56 AR some things can become clear, or clearer. A carbine length gas system and with a conventional carbine H2 action work within a span of reliable function is able to run well with a dedicated can and ammunition, with a properly dedicated port. Some cans and ammunition may need some tweaks in porting to compensate for their issues, normally not by much.
There are some timing issues that are an caused from the base system that unfortunately are not avoidable, but workable solutions.
If one looks at the first example as a baseline, it is just that, a reference, a way to make an appropriate relevance to in a comparison.
Due to the issues of insufficient timing for some events, a slight, but preferable delay may be added to the system by lengthening the distance to the gas port from what it is in the carbine gas conventional system to help compensate a portion of the timing issue. The question is then, how long to extend the gas system. As a reference, a baseline of the first example carbine gas and H2 carbine was used.
Adding a half inch in gas system length resulted in almost no change in the difference in action velocity comparing multiple examples.
Adding one inch resulted in almost no change, except for a select few extremes.
Adding 1.5" makes a larger difference in possible carrier speeds, by a fair margin.
Given the possible span of proper operation, with all of varieties of the ammo, selected can, etc. I would be hard pressed to recommend the mid gas 11.5" suppressed only setup over the conventional carbine, or carbine +1" gas system.
There are other beneficial ways to improve the timing somewhat. A rifle like action buffer and rifle spring actually broaden the range of operation more than the baseline. The results of the rifle like action system with a carbine gas or carbine +1" could be noteworthy.
Barrel weighs 17.10 oz on an old digital food scale. A 7/64 drill bit fit snuggly in the gas port which means its ~0.1094" which matches what the Faxon website specifies (0.110").
My plan is still to send it to Adco to pin the SLR gas block. The good news is that the delayed release means I am pretty close to getting my stamp for the Trek which will live on this barrel and I will not shoot my titanium 7.62 can on this thing.
