AR15.Com Archives
 Why the popularity of 18 inch barrels?
Sid_Post  [Member]
8/13/2006 12:23:30 PM
I have been looking at a lot of uppers that have 18inch barrels. Is this just a compromise between a 20 and 16 inch barrel or, does the 18 inch length offer a lot of ballistic advantage over a 16 inch barrel and very little is gained going up to a 20inch barrel?

Or, in other words why is the 18 inch barrel length so popular? What am I missing?

Thanks,
Sid
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chromeluv  [Member]
8/13/2006 12:27:39 PM

Originally Posted By Sid_Post:
I have been looking at a lot of uppers that have 18inch barrels. Is this just a compromise between a 20 and 16 inch barrel or, does the 18 inch length offer a lot of ballistic advantage over a 16 inch barrel and very little is gained going up to a 20inch barrel?

Or, in other words why is the 18 inch barrel length so popular? What am I missing?

Thanks,
Sid


I have been asking this question alot myself lately, and from my independent research it seems that the 18inch barrel on an ar15 is the most efficient length to get the maximum powder burn and explosion behind the bullet and not compromise weight and maneuverability


Someone with more experience should also weigh in on this...
glock_forty5  [Member]
8/13/2006 12:53:25 PM
I do not know myself. However, in USPSA 3 gun matchs, in general a .223 through 16" barrel does not make minor power factor. All the go fast guys have 18" barrels to make minor and still be easy to move with than a 20". Do not forget the geek factor (if I had more money, I would build a bunch of tricked out ARs too). YMMV

G45
CitySlicker  [Team Member]
8/13/2006 2:39:16 PM
Maybe because the military's SPR uses a barrel of that length.
Hoplophile  [Team Member]
8/13/2006 3:41:56 PM
They are that little bit lighter and handier than a 20" and give more velocity than a 16".

My first 18" was an A2 barrel that I cut back to get rid of the threads during the ban and I found that I really like the feel of it.
john575  [Team Member]
8/13/2006 4:04:46 PM
Bullets tumble better out of that barrel length!!!
AKM  [Member]
8/13/2006 4:25:39 PM
I have never understood why the M16 was fitted with a 20" barrel in the first place.

What other Assault Rifle utilizes a barrel that long?

Sid_Post  [Member]
8/13/2006 4:39:41 PM

Originally Posted By CitySlicker:
Maybe because the military's SPR uses a barrel of that length.


That sort of confirms my couch commando and mall ninja thoughts.
Not_A_Llama  [Member]
8/13/2006 5:44:57 PM
It's like a 16", and it's like a 20", but it's different!!
yekimak  [Team Member]
8/13/2006 5:59:59 PM
because the difference between "ooooh" and "aaaah" is about two inches.
mbinky  [Member]
8/13/2006 6:24:35 PM

Originally Posted By yekimak:
because the difference between "ooooh" and "aaaah" is about two inches.


LMAO!!!
TigerStripe  [Member]
8/13/2006 6:30:25 PM
My guess, with a Phantom or Vortex it's right at 20 inches and doesn't lose the velocity that a 16" does.


TS
GeorgiaBII  [Member]
8/13/2006 6:33:30 PM
With a 1 in 9 barrell 18" is about perfect as you get 2 full rotations out of the bullet. I would say that makes the extra 2" of a 20" unneeded.

Just a thought.


My next build/upgrade/addition is going to be a stainless 18" I think. Been drooling over them for awhile.
Dave_A  [Member]
8/13/2006 6:47:29 PM

Originally Posted By AKM:
I have never understood why the M16 was fitted with a 20" barrel in the first place.

What other Assault Rifle utilizes a barrel that long?



The US military never fully bought the 'assault rifle' concept - even today troops are trained to use the M16 as a precision weapon, and true select-fire is disabled. Contrast this to the Russians, who designed the AK to be used PRIMARILY IN FULL AUTO, with semi as a backup to conserve ammo if there was a shortage....

Back in the 60s, 20" was as short as one could sell, except for 'special purpose' applications....

AyeGuy  [Member]
8/13/2006 7:06:07 PM
FunFact --> SOCOM has specified that the SCAR-L -standard barrel be 13.78" and the -Sharpshooter barrel be 18".

There is prolly a good reason for that...
ASU1911  [Member]
8/13/2006 7:44:10 PM
18 inches is as good as it gets. Anything more and you gain very little velocity. In economics, this concept is called diminishing marginal returns.
CitySlicker  [Team Member]
8/13/2006 9:02:58 PM

Originally Posted By ASU1911:
18 inches is as good as it gets. Anything more and you gain very little velocity. In economics, this concept is called diminishing marginal returns.



Ahh, the point of diminishing returns.


EDIT: You wouldn't happen to have any actual velocity figures would you?
Ironmaker  [Team Member]
8/13/2006 9:10:42 PM
I bought a ban RRA A4 and had it cut to 18" and a Phantom 5C2 flash suppressor put on it. Adco did the work and it turned out great. I went with 18" for the reasons already listed above:
More maneuverable than 20" w/ flash suppressor
Almost as much velocity as a 20" and more than a 16"

I think the full length gas system with an 18" barrel looks cool too.
PALADIN-hgwt  [Team Member]
8/13/2006 10:12:23 PM
I had two inches removed from my MT6700 to eliminate the "HBAR" roll marks after having the barrel turned down to 0.700 under the handguard. Just happened to end up about 18 inches. My old eyes appreciate a rifle length sight radius.

Paladin
Onslaught  [Team Member]
8/13/2006 10:57:08 PM

Originally Posted By CitySlicker:

Originally Posted By ASU1911:
18 inches is as good as it gets. Anything more and you gain very little velocity. In economics, this concept is called diminishing marginal returns.



Ahh, the point of diminishing returns.


EDIT: You wouldn't happen to have any actual velocity figures would you?


Here's MSTN's Velocity Data take it however you like it...

What I see is a 4.5fps per inch gain in the bullet weight I shoot the most, and a 25fps per inch gain in the bullet weight that's the least velocity dependent in the first place.

I wanted to have my 20" cut to 18.5" before I ever joined AR15.com and heard of an "SPR". I did it once, loved it, but sold it for something else.

Now I've got my hands on another 20" barrel, and (if I don't trade it off for a CCW pistol) I will have it done back to 18.5" as well.

Why the extra 1/2"?

Well,
1- that was my original intention so I'm sticking to it
2- so mine's not like everyone else's
3- so I don't look like a Navy "SEEL" wannabe
4- because I'll gain an extra 3/4 fps
5- because I can

Take your pic...
Bloodstripe  [Member]
8/27/2006 9:37:53 AM

Originally Posted By yekimak:
because the difference between "ooooh" and "aaaah" is about two inches.


That's what "she" said
762hurts  [Member]
8/27/2006 11:27:07 AM

Originally Posted By Sid_Post:

Originally Posted By CitySlicker:
Maybe because the military's SPR uses a barrel of that length.


That sort of confirms my couch commando and mall ninja thoughts.


I concur....lol.
AR15barrels  [Dealer]
8/28/2006 4:31:08 AM
I'm suprised that nobody has mentioned the most obvious benefits.
First, you can run a rifle-length gas system.
This is the smoothest running gas system of the big three, carbine, mid and rifle.
Second, 18" is very reliable setup with the rifle gas sytem so you can run anything from Wolf to hot .gov ammo with no problems.

My "3 gunners special" is a stainless 18" barrel, 3/4" under the handguards, rifle gas system and has an MSTN or JP comp on it.
Dave_A  [Member]
8/28/2006 5:12:07 AM

Originally Posted By Sid_Post:
I have been looking at a lot of uppers that have 18inch barrels. Is this just a compromise between a 20 and 16 inch barrel or, does the 18 inch length offer a lot of ballistic advantage over a 16 inch barrel and very little is gained going up to a 20inch barrel?

Or, in other words why is the 18 inch barrel length so popular? What am I missing?

Thanks,
Sid


Because the Army used 18" in their 'SPR' spotter's rifle....
Dave_A  [Member]
8/28/2006 5:14:30 AM
[DELETE]
MSTN  [Industry Partner]
8/28/2006 9:10:20 AM
.... AND FOR THE #1 REASON:



THEY LOOK COOL!!!

WES GRANT
M.S.T.N.
cgv69  [Team Member]
8/28/2006 11:16:31 AM

Originally Posted By AR15barrels:
I'm suprised that nobody has mentioned the most obvious benefits.
First, you can run a rifle-length gas system.
This is the smoothest running gas system of the big three, carbine, mid and rifle.
Second, 18" is very reliable setup with the rifle gas sytem so you can run anything from Wolf to hot .gov ammo with no problems.

My "3 gunners special" is a stainless 18" barrel, 3/4" under the handguards, rifle gas system and has an MSTN or JP comp on it.

Hey Randell,

Steve from ADCO has mentioned in the past the an 18" barrel with a rifle length gas system needed to have the gas part opened up. In his experience, if you don't, you may run into issues when the weather turns cold. Has that been your experience as well and is this something you do on you 18" barrels?
AR15barrels  [Dealer]
8/28/2006 11:22:44 AM

Originally Posted By cgv69:
In his experience, if you don't, you may run into issues when the weather turns cold. Has that been your experience as well and is this something you do on you 18" barrels?


I run my ports ever so much larger than 20" barrels.
I'm from California.
Define cold weather...

Is that when it's like under 60 degrees?
Hoplophile  [Team Member]
8/28/2006 1:31:02 PM
I built this one before I'd ever heard of an SPR. It is simply a very handy size that is very reliable.

cgv69  [Team Member]
8/28/2006 2:02:49 PM

Originally Posted By AR15barrels:

Originally Posted By cgv69:
In his experience, if you don't, you may run into issues when the weather turns cold. Has that been your experience as well and is this something you do on you 18" barrels?


I run my ports ever so much larger than 20" barrels.
I'm from California.
Define cold weather...

Is that when it's like under 60 degrees?

OK, so you made me go look it up. Define cold? Well I never asked him for an exact temp but in this part of the country, cold usually refers to less then 30deg. Steve recommends the port be opened to .105 (not sure what size the standard rifle port is on a 20" barrel?)

Considering you are in Cali and cold is not an issue for you guys, what is your reasoning for opening the gas port "ever so much"?
Onslaught  [Team Member]
8/28/2006 2:11:50 PM

Originally Posted By cgv69:

Originally Posted By AR15barrels:

Originally Posted By cgv69:
In his experience, if you don't, you may run into issues when the weather turns cold. Has that been your experience as well and is this something you do on you 18" barrels?


I run my ports ever so much larger than 20" barrels.
I'm from California.
Define cold weather...

Is that when it's like under 60 degrees?

OK, so you made me go look it up. Define cold? Well I never asked him for an exact temp but in this part of the country, cold usually refers to less then 30deg. Steve recommends the port be opened to .105 (not sure what size the standard rifle port is on a 20" barrel?)

Considering you are in Cali and cold is not an issue for you guys, what is your reasoning for opening the gas port "ever so much"?


What about 18.25" or 18.5" instead of 18"? According to Randall's own FAQ's, an AR15 should have at least 5" of barrel past the gas port for reliable function. Going to 18.25 gives you slightly more than that, so anything beyond should be just bonus, right?
AR15barrels  [Dealer]
8/28/2006 5:30:37 PM

Originally Posted By cgv69:
Steve recommends the port be opened to .105 (not sure what size the standard rifle port is on a 20" barrel?)

Considering you are in Cali and cold is not an issue for you guys, what is your reasoning for opening the gas port "ever so much"?


Typical 20" barrels will be anywhere from 0.089" to 0.096".
That's why some run fine when shortened and some do not.
I generally like the port to be 0.096" to 0.100" and that's where I set them on 18" barrels.
If you were in Alaska, I would probably drill it to 0.105" also.
AR15barrels  [Dealer]
8/28/2006 5:32:44 PM

Originally Posted By Onslaught:
What about 18.25" or 18.5" instead of 18"? According to Randall's own FAQ's, an AR15 should have at least 5" of barrel past the gas port for reliable function.


I'll cut barrels to ANY length.
If you want 17.780", you got it.
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