Okay, I’m not trying to beat a dead horse. I’ve been shooting a little while and I’ve read a massive amount of information on the carbine gas system and can’t seem to get any clarification. Why is a 14.5 barrel best for a carbine? And some people have said that a 14.5 creates more pressure than a 16" carbine.
What’s the story? Is a 14.5 carbine roughly equivalent to a 16" middy as far as pressure, wear and tear, etc?
Thanks for the replies.
IIRC…
The carbine gas system was developed for the shorter CAR rifles with a ~12? Inch barrel. When the army speced the M4 to have a 14.5" barrel they purposely over dwelled the gas system for more reliable operation with a dirty gun.
Actually, 11.5-12.5" is best for a carbine length gas system.
IIRC, the CAR was developed for the Colt Commando with 11.5" barrel.
With any given gas system, a longer barrel has more gas drive available.
“Gas drive” is the area under the pressure curve, PSI x SEC is an appropriate unit.
Also note that pressure is still useful for several milliseconds after the projectile has left the bore.
Compared to a 16" MID, a 14.5 CAR has roughly 35% more gas drive available, while a 14.5 MID has roughly 12% less gas drive available.
As long as plenty of gas drive is available, the gas port size makes the biggest difference in the operation.
For example, a 16" CAR with a .060" port will run much better than a 16" MID with an .093" port.
In this case the CAR is properly gassed and the MID is way over gassed.
Exactly. It’s a system.
By an an absolutely amazing coincidence, the 14.5" barrel uses the standard CAR 15 gas tube and mounts the standard M16 bayonet.
For some reason I’m just having a hard time wrapping my head around what all this means. How do you know if you have the right size gas port for the barrel? And does the 14.5 car put more wear and tear than a 16 car?
Thanks for your patience everyone.
I am pretty sure that I have posted what are considered correct port sizes IAW the TDP and companies in the industry that know what they are doing.
Don’t worry, it’s simply sophisticated. 
Getting proper gas ports generally requires sticking to a handful of quality manufacturers, as many of the 2nd tier barrels are over gassed.
Knowing if it’s right involves measuring the size and comparing it to known good numbers.
All things being equal, the 14.5" will have less gas drive than a 16".
If the gas ports are properly sized for each barrel, they will have nearly identical gas drive.
It appears that odugrad has read so much on the subject that he is now thoroughly confused. Let’s see if I can clarify things.
His Question: Why is a 14.5" barrel best for a carbine? Answer: The standard bayonet fits.
His question: How do you know if you have the right size gas port for the barrel? Answer: If it was made by a reputable manufacturer.
His question: Does a 14.5 car put more wear and tear than a 16 car? Answer: No.
My personal suggestion to odugrad is not to worry about any of this. However, there are NFA issues with the 14.5. Just buy one that you like and enjoy it!
One hitch here. Define “reputable”. People think DPMS, Bushmaster, Whogivesaphuck Arms and others are reputable. So that needs to be more defined.
So as long as the port is properly sized there shouldn’t be a difference in pressure or wear and tear?
I have a DD M4A1 and a DDM4V5. Should they be equivalent as far as pressure?
I have another question.
What is considered the optimum gas system for barrel lengths 14.5, 16, and 10.3?
What is considered overgassed and what would produce the least wear and tear?
Thanks so much.
16" = .60" I believe
14.5" = .062"
10.3" = .071".
If we are talking carbine length gas systems.
16" and 14.5" barrels from most companies (Colt as an example) use the same .063 gas port.
And why is a 14.5 carbine better than a 16"?
Does the longer dwell time on the 16" reduce the pressure? Wouldn’t that be less harsh on the rifle?
This sentence made it click for me…Finally.
Shorter barrels need larger ports to move more gas because there is less time (barrel length) before bullet leaves the barrel venting all gas. And vice-versa. Right?
And buffer/spring weights are used to compensate back to good operation with improper ports? Finally may be getting it.
Now I possibly know why my first used AR a decade ago was a jamomatic, but my current high quality choice functions flawlessly out of the box.
If I add a suppressor and not a switch block increaseing length, gas would increase, so would I just change buffer/spring to bring back normal operation? Is there a formula or just trial and error?
Noah
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