AR-15 18" LW Rifle Length Bbl?


February 26, 2008, 02:01 AM
(Formerly AR-15 18" LW Rifle Length Bbl?)
After much reading here I'm thinking an 18" barrel with rifle length gas system would be great for my next AR build. My only problem is that I'd like to stay with a lightweight barrel. Everywhere I've looked the 18" bbls are either heavy or midweight. So does an 18" rifle length gas system light weight or Govt. profile barrel exist? If so, please tell me who makes it. Thank y'all.

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Zak Smith
February 26, 2008, 12:51 PM
Any of the custom barrel makers will do it.

My 3Gun rifle was built by MSTN from a Krieger blank.


February 26, 2008, 01:09 PM
Or anyone that builds ARs. I just get regular 20" rifle barrels, cut, crown, and thread.

February 26, 2008, 01:13 PM
Thanks Zak. I also need to call Sabre Defence today. Their Competition Deluxe and Extreme Barrel Assemblies are is listed as "Fluted Mid-Weight Mid-Length, Fitted with M4 Feed Ramps, 410 Stainless Fluted Barrel". They offer them in 16, 18, and 20 inch lengths. Now the question is whether the 18 inch bbls use the rifle or mid length gas system. I may still be looking at custom makers.

February 26, 2008, 09:43 PM
Are there any known issues with 16" bbls running rifle length gas systems? Any major advantage to an 18" bbl with a rifle length gas system over a 16" bbl with a rifle length gas system?

February 26, 2008, 09:51 PM
I'm wanting the same rifle. My plan is to order the CMMG LW 20" and have it chopped by someone reputable after playing with it for a while.

February 26, 2008, 09:52 PM
i could be wrong, but i don't think 16" will support a rifle-length gas system. the gas port is too close to the muzzle.

Zak Smith
February 26, 2008, 10:00 PM
Sabre makes 18's with midlengths. IMO, these should be RL for best operation. some 16's run with RLGS and some don't. Most people recommend against it. I consider 17" the shortest to go with a RLGS. As far as cold-weather concerns, I shot my 17" 3gun rifle at -5*F with mild commercial ammo and it was fine.


February 26, 2008, 10:02 PM
why not just order the CMMG 18"?

February 27, 2008, 03:41 AM
why not just order the CMMG 18"?
Pete, I think Bratch and I are in the same position. I don't want CMMG's standard 18" bbl because it's a midweight, and I want a lightweight.

A 20" LW with a chop to 18 as you suggested may be a possibility for me.

Zach S
February 27, 2008, 08:57 AM
You could get a CMMG 18" bbl ($250) and have it turned down by adco ($64).

March 7, 2010, 11:35 PM
Breathing in new life to this thread.

I am looking for a 18" 1x8 light weight barrel and wanted to know what are the best options other than Sabre for $450.
DPMS Mini-SASS $199 but weighs 3.65 lbs
BCM is Mid Weight
YHM doesnt do 18"
CMMG no go for 18"

March 8, 2010, 12:08 AM
SSGTMUSMC Are you sure CMMG doesn't have an 18"?

Last I looked at their catalog they had one of everything and many things I hadn't even considered before. I think you should double check with them.

March 8, 2010, 10:31 AM
From what I've studied it's not what length fits from the chamber, but from the muzzle. The timing for the bullet to pass the gas port to the muzzle determines the "length" of the gas system.

Taking a longer look at barrels shows the length from port to muzzle needs to be 5 to 7 inches.

Another way to look at it is what length barrel is needed for the intended results. The extra length of 18 to 20 inch barrels nets better 400 yard performance, and mid/rifle gas systems just come with them. Taking either to cut down to 16 ignores the timing, and if it's a government profile, sometimes leaves the wrong diameter for the gas block.

Like a lot of other stuff, it's not just a cut and try thing.

March 9, 2010, 06:50 PM
Length effects timing. But it also effects the power of the gas in the tube. That powder gas can only expand so much before it starts loseing some of its pressure. That is why carbine length systems can feel aggressive without a h buffer or h2 buffer.

Going to a full length system in an 18" my cause you to short stroke when you use weaker ammo. Mid length is best for 18" IMO. 16" is good in middy or carbine, never rifle length.

Zach S
March 10, 2010, 08:01 AM
Going to a full length system in an 18" my cause you to short stroke when you use weaker ammo.
Simple solution: dont use crap ammo.

March 10, 2010, 09:11 AM
The ammo isn't so much crap as the inherent restriction of a gas system - DI or Piston. It's tuned to a specific narrow range of pressure. What would be no problem for a lever or bolt gun shooting a wide diversity of loads is for a gas gun. Semi auto shotguns are the same, look at all the fancy methods used to get one to shot light loads to heavy magnums.

Semi autos are stuck with a narrower operating range of acceptable loads. Some auto pistols are so finicky they are developed around one only - the Rohrbaugh .380 comes to mind. Shooting an AR designed for military NATO rounds with lower power bolt action plinking rounds doesn't make it crap ammo, just a misapplication.

Zak Smith
March 10, 2010, 12:07 PM
My 3Gun rifles are 17"'s with RLGS. They run fine even with "weak" ammo and sub 10-degree (F) temperatures.

March 10, 2010, 03:11 PM
I was looking for a 18" lightweight barrel for my latest build...couldn't find one except for custom makers. I finally bought an SPR style SS barrel with RLGS from Bravo Company and sent it to ADCO to have it turned down to a LW profile. Machining only cost ~$70 and it was done well, with fast turnaround.

This rifle only tips the scales at around 6.5 lbs. I like it.

March 10, 2010, 07:13 PM
Just a thought. If you shoot long range a 20" barrel is best. If you shoot CQC 14 is best with 16 next. Will an 18" fill some gap?

FYI try looking at Whiteoak barrels. They have 18" barrels, but I'm not sure if they are in the profile you're looking for.

August 22, 2010, 11:46 PM
my neighbors DPMS is a 16'' inch barrel, with rifle gas.

silly thing shoots everything we feed it just fine.

i do think dpms opens the gas port a wee bit, but it works, and works very well.

p.s. i sent a cmmg middy weight barrel to ADCO, and he moved the gas port to mid-length,

and trimmed her down to light weight, its one sweet rifle,

cannot wait to build a rifle gas dissy .


August 22, 2010, 11:50 PM
oh, and adco's turn around was ...FAST !!! :)

im already thinking of maybe sending adco my 16'' M4 barrel, move gas port to rifle.

that would be very lightweight.

AR-15 Rep
August 23, 2010, 03:18 AM
Something to think about....
A while back there were some dissipators out there that took a 20" barrel and RLGS and cut them down to 16" to make them into dissipators, they had a reliability problem. Most manufacturers learned this mistake and went back to the LP Gas block under the handguards and put a front sight or gas block in front just for the sight radius.
Now we skip ahead a few years and people are talking about doing the same but putting a larger gas port to make up for the short time to pass the gas through the system to operate the rifle. Hmmmm with that short of time to pass enough pressure would mean the gas tube is now under alot higher gas pressure for the rifle to operate.
Now the question is how much gas pressure can that tube hold and at what temp? and also can the gas rings in the BCG take the pressure for that shorter burst?
Does the projectile now have a scar on it from the larger port? Will it affect accuracy?
Those are just some things that come to mind and maybe they looked into them but something to think about.

August 23, 2010, 08:31 AM
IIRC Noveske offers the intermediate gas for an 18" to keep the timing and gas port within the window. Only aftermarket tube/rails seem to be available for it, which is usually no big deal.

While there certainly are good carbines and rifles out there that have been made to run with different loads, they were factory assembled. Changing the gas system length sometimes has unintended consequences. High speed videos of the various combinations show the entire action has to be thought thru. Even then cycling may be reliable, but funny things are going on.

AR's may be like Legos, just like Chevy's, but changing the displacement and induction create situations where the car starts and runs fine, but has driveability issues and becomes a royal pain. The AR is a lot simpler, but a thorough understanding of the entire gas cycle needs to be at hand. What each part does in the cycle is important to overall smooth running.

Case in point, carbine length gas was developed for military use and 14.5" barrels. It doesn't translate well to 16", which is exactly why the industry offers midlength now. It just works better. It's no accident there are a number of different weight buffers, either. One size does not fit all, and one simple change can mean making quite a few more.

If an 18" is assembled by a competent maker, all the correct parts are installed day one. As a change to an existing rifle, it may take months and quite a few parts gone through to discover a working combination - which still may not be the optimum one.

Just food for thought.

August 23, 2010, 08:36 AM
I have one of these:

Works great. Deosn't weigh much more than my 16" carbine. I put a big scope on it which deos weight alot and really made it an SPR, but with a Aimpont or Eotech it's seems fine.

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