View Full Version : ar-15 barrels.

January 17, 2010, 12:27 AM
I am looking for an custom 18" ar-15 barrel.

I don't know much about the midlength configuration in respects to the gas tube location. Do I need to get a 18" barrel with a carbine length gas tube?

January 17, 2010, 12:32 AM
Do I need to get a 18" barrel with a carbine length gas tube?
If that's what you want, then sure. But I wouldn't. Assuming we're talking 5.56, I would (and did) get mid-length gas on an 18" barrel.

January 17, 2010, 12:39 AM
Ok, what barrel makers make such a barrel?

January 17, 2010, 12:47 AM
Mine was RRA. White Oak has them, too. CMMG. They're really not rare.

January 17, 2010, 02:23 AM
Del-Ton or M&A Parts is who I am probably going to go with for my next upper...

January 17, 2010, 08:50 AM
Midlength would be preferable to carbine

Some companies make a "intermediate length"


Bartholomew Roberts
January 17, 2010, 09:08 AM
On an 18" barrel, I would go with a rifle length gas system. My personal chart is:

10.5"-14.5" = carbine gas
14.5-16.5" = midlength gas
17"* and up - rifle gas

*The 17" is only if MSTN builds it; otherwise 18"

January 17, 2010, 10:50 AM
Sabre Defense's barrels are notoriously accurate as can be.


And as Mr. Roberts pointed out, you'll be best off with rifle-length gas on an 18".

January 17, 2010, 11:28 AM
Can you guys tell me why the rifle length gas tube is better on an 18"?

January 17, 2010, 03:07 PM
The exhaust gasses spend more time pushing the round, so it goes farther, more accurately.

It also spends more time traveling back to the bolt to cycle it, which makes the round more accurate by giving it more time to leave the barrel before the weapon starts jerking around from cycling the bolt.

January 17, 2010, 03:14 PM
I have a CMMG 18" SPR barrel with the rifle length gas system. It's very accurate and very smooth cycling (very reliable too).

Your choices will be rifle, intermediate (on those who offer it) and midlength. Midlength is likely to be a bit overgassed, with rather strong extraction and ejection, sometimes overly strong. Rifle might be a bit undergassed (mine isn't), but that's easily enough handled. Intermediate might be the best pick, but t'were I to do that, I'd be likely to go with a low profile gas block because handguards will be hard to come by- the low profile block will let you use a rifle length free float handguard, increasing options.

Bartholomew Roberts
January 17, 2010, 04:09 PM
In the AR15 gas system, there are three big factors that affect the operation:

1. The diameter of the gas port
2. The amount of barrel before the gas port
3. The amount of barrel after the gas port (dwell time)

When you move the gas port back, you are tapping the gas where it is higher pressure. For example, the port pressure with a rifle length gas system is between 12,000-15,000psi. With a carbine length gas system, the port pressure is about 24,000-30,000psi.

The shorter travel time (for the gas to reach the port, go through the gas tube and begin unlocking the bolt) also means that the unlocking cycle begins faster, contributes to a higher cyclic rate and generally narrows the window where the rifle will function reliably.

The second issue is dwell time. When you have more barrel past the gas port than a 20" AR, the gas pressure flows through the gas port for a longer period (because it takes a little longer for the bullet to exit and the pressure to drop off). The result is a rifle that is overgassed for the design (i.e. a 16" barrel with carbine gas); but you can also have too little past the muzzle.

However, if you have too little barrel after the gas port, the gas pressure drops off before you can get enough gas to cycle the bolt. 10.5" ARs are hard to get running reliably because you have the combination of the shorter gas system AND a shorter dwell time to deal with.

All of the numbers in an AR (cyclic rate, bolt load, etc.) are designed around the idea that you have a 20" barrel with rifle length gas system. The further you get away from that, the narrower the window gets for reliability.

An 18" with a midlength gas system is going to be taking in gas at a higher pressure (shorter than rifle gas system) and it is going to have a slightly longer dwell time than a 20" rifle. The 18" with the rifle length gas system has the original design's gas system; but a little shorter dwell time that you can compensate for by opening up the port a bit.

January 18, 2010, 01:25 PM

December 8, 2012, 01:46 AM
I know exactly what the problem is - its an AR 15!

Sorry, I couldn't resist...

Seriously though, mags are always the most likely culprit. There is an element of truth to my first comment too, but constantly jamming when the weapon isnt that dirty isnt a "normal" f2f or f2e issue, something is going on.

December 8, 2012, 12:22 PM
You raised a three year old thread from the dead to say that? :D

December 9, 2012, 12:04 AM