Firearm & Gun Forum - FireArmsTalk.com > Long Guns > Auto & Semi-Auto Discussion > AR-15 Discussion > Barrel Length Question

Reply
 
LinkBack Thread Tools Display Modes
Old 08-04-2011, 12:50 AM   #1
Feedback Score: 0 reviews
 
Schmeekey's Avatar
 
Join Date: Oct 2010
Location: Somewhere,Colorado
Posts: 27
Default Barrel Length Question

If a carbine gas system can accommodate a 10.5 inch barrel why is it that the shortest a mid-length can accommodate is a 14.5?



__________________

Psalms 18:30-35
It is God who arms me with strength and makes my way perfect. He trains my hands for battle; my arms can bend a bow of bronze. You give me your shield of victory, and your right hand sustains me; you stoop down to make me great.

Schmeekey is offline  
 
Reply With Quote

Join FirearmsTalk.com Today - It's Free!

Are you a firearms enthusiast? Then we hope you will join the community. You will gain access to post, create threads, private message, upload images, join groups and more.

Firearms Talk is owned and operated by fellow firearms enthusiasts. We strive to offer a non-commercial community to learn and share information.

Join FirearmsTalk.com Today! - Click Here


Old 08-04-2011, 12:59 AM   #2
Moderator
FTF_MODERATOR.png
Feedback Score: 0 reviews
 
JonM's Avatar
 
Join Date: Nov 2009
Location: Rochester WI,Rochester WI
Posts: 16,241
Liked 4483 Times on 2365 Posts
Likes Given: 255

Default

because 14.5 is about midway tween 10 and 20 hence the label mid length. carbine length wasnt meant for a 16inch barrel its meant for an actual M4 military barrel which is 14.5 inches the extra length 4.5 at the end is for a bayonet.

16 inch barrels would perform better with a midlength giving a smoother gas impulse and longer sight radius.



__________________

"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." — L. Neil Smith

The problem with being stupid is you cannot simply decide to stop doing dumb things...

JonM is offline  
 
Reply With Quote
Old 08-04-2011, 02:11 AM   #3
Feedback Score: 0 reviews
 
Schmeekey's Avatar
 
Join Date: Oct 2010
Location: Somewhere,Colorado
Posts: 27
Default

Quote:
Originally Posted by JonM View Post
because 14.5 is about midway tween 10 and 20 hence the label mid length. carbine length wasnt meant for a 16inch barrel its meant for an actual M4 military barrel which is 14.5 inches the extra length 4.5 at the end is for a bayonet.

16 inch barrels would perform better with a midlength giving a smoother gas impulse and longer sight radius.
Thanks for the answer, but that wasn't really what I was asking. I was wondering why a carbine length can semi reliably run a barrel that ends one inch away from the gas block. By that logic, you would think that a middy could do the same. But as I have been told, this is not possible and that the shortest you can have is 14.5. I am simply wondering why you can't use a 12.5" barrel for example. I hope this clarified my question.
__________________

Psalms 18:30-35
It is God who arms me with strength and makes my way perfect. He trains my hands for battle; my arms can bend a bow of bronze. You give me your shield of victory, and your right hand sustains me; you stoop down to make me great.

Schmeekey is offline  
 
Reply With Quote
Old 08-04-2011, 02:24 AM   #4
FTF_SUPPORTER.png
Feedback Score: 0 reviews
 
Gatoragn's Avatar
 
Join Date: Dec 2010
Location: North GA. Back home in Dixieland.
Posts: 3,205
Liked 1008 Times on 566 Posts
Likes Given: 393

Default

I don't know if this is relevant to your question or not.

According to the owner of my favorite LGS, the shortest legal rifle barrel for civilian use is 16 inches. 14.5 inch barrels accomplish this by having a muzzle device (flash hider, brake) pinned or welded to meet the 16 inch minimum.

To be a legal rifle, a 12.5 inch barrel would need a 3.5+ inch muzzle device.

__________________
Gatoragn is offline  
 
Reply With Quote
Old 08-04-2011, 02:43 AM   #5
Devil's Advocate and Provocateur
FTF_MODERATOR.png
Feedback Score: 0 reviews
 
TekGreg's Avatar
 
Join Date: Jul 2011
Location: Cincinnati,OH
Posts: 1,720
Liked 1139 Times on 610 Posts
Likes Given: 531

Default

Quote:
Originally Posted by Schmeekey View Post
Thanks for the answer, but that wasn't really what I was asking. I was wondering why a carbine length can semi reliably run a barrel that ends one inch away from the gas block. By that logic, you would think that a middy could do the same. But as I have been told, this is not possible and that the shortest you can have is 14.5. I am simply wondering why you can't use a 12.5" barrel for example. I hope this clarified my question.
Schmeekey, even though the short barrel terminates one inch from the gas block, the distance the gas has to travel back in the gas system is much less in the short barrel system. As you lengthen the barrel AND the gas tube, the bullet must remain in the barrel longer past the gas block to pressurize the longer tube to reliably cycle the gas system. That extra couple of inches allows time for the gas to cycle back down the longer tube and do all that it needs to do! I know that manufacturers test different lengths and sometimes cannot get certain lengths to function because of recoil/gas pulse problems based on length, tube size, gas block, etc. Gas systems are a constant compromise and balancing act, and that is probably why you are not seeing the "reasonableness" of this build. You may be able to change the tube and/or the block and get a better result from a custom build.

I hope that helped!
__________________

.
.
“Malo periculosam, libertatem quam quietam servitutem." (I prefer the tumult of liberty to the quiet of servitude.). Thomas Jefferson

"All that is necessary for the triumph of evil is that good men do nothing."

-Edmund Burke, Loosely translated from Thoughts on the Cause of Present Discontents. (1770)

TekGreg is offline  
 
Reply With Quote
Old 08-04-2011, 03:04 AM   #6
Moderator
FTF_MODERATOR.png
Feedback Score: 0 reviews
 
JonM's Avatar
 
Join Date: Nov 2009
Location: Rochester WI,Rochester WI
Posts: 16,241
Liked 4483 Times on 2365 Posts
Likes Given: 255

Default

the closer the port is to the chamber the higher the gas pressure thats why carbine ports are smaller than rifle 20" ports. takes less gas to operate the carbine tubes due to the increased pressure

__________________

"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." — L. Neil Smith

The problem with being stupid is you cannot simply decide to stop doing dumb things...

JonM is offline  
 
Reply With Quote
Old 08-04-2011, 04:26 AM   #7
Feedback Score: 0 reviews
 
Schmeekey's Avatar
 
Join Date: Oct 2010
Location: Somewhere,Colorado
Posts: 27
Default

Quote:
Originally Posted by TekGreg View Post
Schmeekey, even though the short barrel terminates one inch from the gas block, the distance the gas has to travel back in the gas system is much less in the short barrel system. As you lengthen the barrel AND the gas tube, the bullet must remain in the barrel longer past the gas block to pressurize the longer tube to reliably cycle the gas system. That extra couple of inches allows time for the gas to cycle back down the longer tube and do all that it needs to do! I know that manufacturers test different lengths and sometimes cannot get certain lengths to function because of recoil/gas pulse problems based on length, tube size, gas block, etc. Gas systems are a constant compromise and balancing act, and that is probably why you are not seeing the "reasonableness" of this build. You may be able to change the tube and/or the block and get a better result from a custom build.

I hope that helped!
Thanks! this totally answered my question. I was just curious to why no one had made a shorty mid-length. Thanks for such a detailed answer.
__________________

Psalms 18:30-35
It is God who arms me with strength and makes my way perfect. He trains my hands for battle; my arms can bend a bow of bronze. You give me your shield of victory, and your right hand sustains me; you stoop down to make me great.

Schmeekey is offline  
 
Reply With Quote
Old 08-04-2011, 02:13 PM   #8
Devil's Advocate and Provocateur
FTF_MODERATOR.png
Feedback Score: 0 reviews
 
TekGreg's Avatar
 
Join Date: Jul 2011
Location: Cincinnati,OH
Posts: 1,720
Liked 1139 Times on 610 Posts
Likes Given: 531

Default An even better response

Schmeekey, I did some more research and found out some more information for you.

As the bullet is fired, the brass casing is under extremely high pressure and is locked into the chamber. As you know, when the bullet passes the gas port, escaping gasses go up into the gas tube and back toward the bolt and bolt carrier, forcing them to unlock and move back to cycle the action. However, if the bullet is still in the barrel when the bolt tries to unlock because there is a lot of barrel length it still has to travel before it exits, then the case still has too much pressure to release the chamber and the gun may not cycle. The opposite is also a problem when the barrel is very short. If the bullet exits the barrel too soon after passing the gas port, then you have too little dwell time and insufficient gas going down the tube back into the bolt and bolt carrier (since more more gasses end up escaping out of the end of the barrel). This also causes malfunctions. Again, this can be adjusted in a number of ways, size of gas port, size of gas tube, position of gas block, length of tube, etc.

This is the best answer I could get for you at the moment. Stay tuned for further updates if I come up with something even more detailed - I may call Colt and talk to their gas specialist.

__________________

.
.
“Malo periculosam, libertatem quam quietam servitutem." (I prefer the tumult of liberty to the quiet of servitude.). Thomas Jefferson

"All that is necessary for the triumph of evil is that good men do nothing."

-Edmund Burke, Loosely translated from Thoughts on the Cause of Present Discontents. (1770)

TekGreg is offline  
 
Reply With Quote
Old 08-04-2011, 06:46 PM   #9
Feedback Score: 0 reviews
 
Schmeekey's Avatar
 
Join Date: Oct 2010
Location: Somewhere,Colorado
Posts: 27
Default

Quote:
Originally Posted by TekGreg View Post
Schmeekey, I did some more research and found out some more information for you.

As the bullet is fired, the brass casing is under extremely high pressure and is locked into the chamber. As you know, when the bullet passes the gas port, escaping gasses go up into the gas tube and back toward the bolt and bolt carrier, forcing them to unlock and move back to cycle the action. However, if the bullet is still in the barrel when the bolt tries to unlock because there is a lot of barrel length it still has to travel before it exits, then the case still has too much pressure to release the chamber and the gun may not cycle. The opposite is also a problem when the barrel is very short. If the bullet exits the barrel too soon after passing the gas port, then you have too little dwell time and insufficient gas going down the tube back into the bolt and bolt carrier (since more more gasses end up escaping out of the end of the barrel). This also causes malfunctions. Again, this can be adjusted in a number of ways, size of gas port, size of gas tube, position of gas block, length of tube, etc.

This is the best answer I could get for you at the moment. Stay tuned for further updates if I come up with something even more detailed - I may call Colt and talk to their gas specialist.
Thank you again for putting in time to answer my question.
__________________

Psalms 18:30-35
It is God who arms me with strength and makes my way perfect. He trains my hands for battle; my arms can bend a bow of bronze. You give me your shield of victory, and your right hand sustains me; you stoop down to make me great.

Schmeekey is offline  
 
Reply With Quote
Old 08-04-2011, 09:53 PM   #10
Feedback Score: 0 reviews
 
Quentin's Avatar
 
Join Date: Apr 2010
Location: Idaho
Posts: 6,246
Liked 731 Times on 542 Posts
Likes Given: 699

Default

Great answers, TekGreg!

Just repeating some of the points, you probably can get a midlength gas system to work with a barrel shorter than 14.5" but it would take a lot of tuning to make it reliable with various ammo, increasing the gas port and playing with the buffer weight and spring, maybe even a light weight bolt carrier. Even in a 14.5" sometimes it's hard to get Wolf and other light powered ammo to be reliable with midlength gas.

It's just not worth it so you see the carbine length gas in shorter barrels.



__________________

The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
________________________________________
US Army 1966-69, VFW Life Member, Retired Geek

Quentin is offline  
 
Reply With Quote
Reply

Thread Tools
Display Modes


Similar Threads
Thread Thread Starter Firearms Forum Replies Last Post
Best barrel length for .75? Scratchammo Blackpowder & Musket 6 05-21-2011 11:26 PM
barrel twist and barrel length Phadil Gunsmithing Forum 13 09-22-2010 04:58 PM
AR-15 Carbine Length Handguard on Full Length Barrel hopps402 Gunsmithing Forum 9 03-03-2010 03:54 PM
Ruger barrel length question, yes or no. SoL .22 Rifle/Rimfire Discussion 6 08-15-2009 01:45 PM
Best Barrel Length jeepcreep927 Ammunition & Reloading 5 08-22-2008 02:27 AM