Carbine vs midlength gas on AR?

THORN74 · October 18, 2014, 08:53 PM

So I was just watching a YouTube video on the subject and something occured to me....

Given the two rifles identical but the gas system , say 16" barrel wouldn't the mid length get just a bit more velocity out of the same ammo?

I'm thinking that with the gas tap further down the barrel the midlenght would allow for maximum pressure to be applied to the projectile for a longer duration. Meaning longer period of acceleration. Even if the bullet is accelerating the full length of both barrels the mid length has full pressure longer.

What do u guys think ?

Metal god · October 18, 2014, 10:37 PM

Well that's not where I though this thread was going but I think that's an interesting thought and is in some way true . How ever I'm not sure if it would be enough to notice or matter . A couple months ago I did a test that confirmed how you hold the rifle can change velocity as much as 60fps with my 308 . I can't imagine the bleed off difference would transfer to a velocity difference more then how you hold the rifle from shot to shot . Meaning there would be other factors that could be causing your velocity differences and it would be hard to confirm that the gas port distance is what's causing any velocity changes .

That all being said , I have both gas systems and I'll run some test soon to see if there is a difference . Not sure when I'll get to it but will at some point .

EDIT: just got to think that for the test to work the rifles would need to be the same with the only difference being the gas system . I do not have 2 of the same rifles really . They either weigh different amounts by a # or 2 , have differnt twist or crome lined and non-crome lined barrels . All of which I believe could change your velocity by 25 to 50fps . I'll still give it a try but don't think my results will mean much .

G.barnes · October 19, 2014, 06:27 AM

A carbine gas system is more abusive to internals. I don't think the velocity difference would be worth the extra wear and tear.

reynolds357 · October 19, 2014, 06:52 AM

The slight velocity advantage would go to the Mid length. This is assuming you have barrels the same length. The M4 length gas system really makes very little sense with a 16" barrel, but all the manufacturers are doing it.

Ridge_Runner_5 · October 19, 2014, 04:52 PM

Midlength is easier on the internals, has less felt recoil, provides more grip area and a longer sight radius.

Carbine is "mil-spec" and you can mount a grenade launcher if you so desire.

Midlength also lets you mount a bayonet and have it sit correctly.

Mobuck · October 19, 2014, 07:33 PM

"just got to think that for the test to work the rifles would need to be the same with the only difference being the gas system . I do not have 2 of the same rifles really "
Even barrels of same maker will have velocity variances exceeding the difference between carbine and mid-length gas systems. The only way to make a comparison would be to have both gas ports on the same barrel and plug one while testing the other.
Honestly, I can't say I've noticed ANY difference between carbine and mid-length gas systems. I'm not a "high volume" AR shooter but I have a SIL who did quite a bit of "high volume" fire with an M-4 in Iraq. Use a good buffer /spring and properly sized gas port and you won't have much trouble. All that said, I prefer the mid-length since it's somewhat more forgiving to less than optimum port sizing and mix and match parts.

SVTCobra306 · October 19, 2014, 08:53 PM

The difference in velocity is miniscule, however the other benefits of the midlength gas system on a 16" barrel by themselves justify using it.. and yes, the slight difference should favor the midlength system as well.

THORN74 · October 20, 2014, 08:51 AM

Should have mentioned in my original post. I already have a midlenght, for some time now. So while I appreciate the other info, I was really just concerned with the theory I proposed.

I am well aware of the other advantages, its why I went mid length in the first place.

MarkCO · October 20, 2014, 08:56 AM

Theoretically, there is no difference. The carbine port is at a higher pressure than the midlength port, but of a smaller diameter. If sized correctly, both "should" extract the same amount of energy from the system and if everything else is equal, there will be no difference.

In practice, I could not find any difference in velocity between two barrels made from the same blanks with the only difference being the gas port location.

btmj · October 21, 2014, 11:24 PM

I have both. From a shooting standpoint, the mid=length seems to have a slightly softer recoil... but the gas length does not make nearly as much difference as a heavy barrel... An 16" HBAR barrel adds over a pound to the rifle compared to a 16" M4 profile, and that makes more difference in recoil than the gas length.

mxsailor803 · October 23, 2014, 07:40 AM

To the OP, in theory, you "should" gain a little FPS. Now thats just in theory. I don't even know if it would even be a big enough difference to worry about. Even with handloads, its next to impossible to get the exact FPS repeatable enough times to accurately measure a difference. Example, you want to do a 5 shot average FPS, now you will need 10 rounds to accurately measure the difference. I'm not saying its impossible to get a exact measurement, just won't be easy or (for me at least) justifiable difference.

tirod · October 23, 2014, 09:54 AM

With Carbine gas, to be milspec it came only in a 14.5" barrel, and that is a notable subject of discussion from the events in Afghanistan. Many considered ti to have too little effective range in the long distance shooting wars there.

So was a 16" barrel. The Army went back to a 20" one per squad, refitted the M14 (the few Clinton didn't have demilled,) and started supporting patrols with crew served weapons.

Barrel length has more impact that where the gas is tapped. And even on the same length barrel overall, the gas port size is different. That also effects the results.

All said, too many variables to make a comparison when the ammo loading process and rifling itself are more likely to affect the results.

October 18, 2014, 08:53 PM	#1
THORN74 Senior Member Join Date: May 30, 2010 Location: Chicago 'Burbs Posts: 543	Carbine vs midlength gas on AR? So I was just watching a YouTube video on the subject and something occured to me.... Given the two rifles identical but the gas system , say 16" barrel wouldn't the mid length get just a bit more velocity out of the same ammo? I'm thinking that with the gas tap further down the barrel the midlenght would allow for maximum pressure to be applied to the projectile for a longer duration. Meaning longer period of acceleration. Even if the bullet is accelerating the full length of both barrels the mid length has full pressure longer. What do u guys think ?

October 18, 2014, 10:37 PM	#2
Metal god Senior Member Join Date: April 10, 2012 Location: San Diego CA Posts: 6,876	Well that's not where I though this thread was going but I think that's an interesting thought and is in some way true . How ever I'm not sure if it would be enough to notice or matter . A couple months ago I did a test that confirmed how you hold the rifle can change velocity as much as 60fps with my 308 . I can't imagine the bleed off difference would transfer to a velocity difference more then how you hold the rifle from shot to shot . Meaning there would be other factors that could be causing your velocity differences and it would be hard to confirm that the gas port distance is what's causing any velocity changes . That all being said , I have both gas systems and I'll run some test soon to see if there is a difference . Not sure when I'll get to it but will at some point . EDIT: just got to think that for the test to work the rifles would need to be the same with the only difference being the gas system . I do not have 2 of the same rifles really . They either weigh different amounts by a # or 2 , have differnt twist or crome lined and non-crome lined barrels . All of which I believe could change your velocity by 25 to 50fps . I'll still give it a try but don't think my results will mean much . __________________ If Jesus had a gun , he'd probably still be alive ! I almost always write my posts regardless of content in a jovial manor and intent . If that's not how you took it , please try again . Last edited by Metal god; October 19, 2014 at 12:35 AM.

October 19, 2014, 06:52 AM	#4
reynolds357 Senior Member Join Date: December 10, 2012 Posts: 6,165	The slight velocity advantage would go to the Mid length. This is assuming you have barrels the same length. The M4 length gas system really makes very little sense with a 16" barrel, but all the manufacturers are doing it. Last edited by reynolds357; October 19, 2014 at 08:16 PM.

October 19, 2014, 08:53 PM	#7
SVTCobra306 Senior Member Join Date: March 18, 2013 Posts: 434	The difference in velocity is miniscule, however the other benefits of the midlength gas system on a 16" barrel by themselves justify using it.. and yes, the slight difference should favor the midlength system as well. __________________ Former US Army Paratrooper (1/509th PIR and 2nd BCT 82nd ABN DIV) 2 time OIF Veteran NRA Certified RSO/CRSO NRA Certified Pistol Instructor

October 20, 2014, 08:56 AM	#9
MarkCO Senior Member Join Date: October 21, 1998 Location: Colorado, USA Posts: 4,308	Theoretically, there is no difference. The carbine port is at a higher pressure than the midlength port, but of a smaller diameter. If sized correctly, both "should" extract the same amount of energy from the system and if everything else is equal, there will be no difference. In practice, I could not find any difference in velocity between two barrels made from the same blanks with the only difference being the gas port location. __________________ Good Shooting, MarkCO www.CarbonArms.us

October 19, 2014, 06:27 AM	#3
G.barnes Senior Member Join Date: June 3, 2011 Posts: 182	A carbine gas system is more abusive to internals. I don't think the velocity difference would be worth the extra wear and tear.

October 19, 2014, 04:52 PM	#5
Ridge_Runner_5 Senior Member Join Date: February 8, 2008 Location: Denver, CO Posts: 1,925	Midlength is easier on the internals, has less felt recoil, provides more grip area and a longer sight radius. Carbine is "mil-spec" and you can mount a grenade launcher if you so desire. Midlength also lets you mount a bayonet and have it sit correctly.

October 19, 2014, 07:33 PM	#6
Mobuck Junior member Join Date: February 2, 2010 Posts: 6,846	"just got to think that for the test to work the rifles would need to be the same with the only difference being the gas system . I do not have 2 of the same rifles really " Even barrels of same maker will have velocity variances exceeding the difference between carbine and mid-length gas systems. The only way to make a comparison would be to have both gas ports on the same barrel and plug one while testing the other. Honestly, I can't say I've noticed ANY difference between carbine and mid-length gas systems. I'm not a "high volume" AR shooter but I have a SIL who did quite a bit of "high volume" fire with an M-4 in Iraq. Use a good buffer /spring and properly sized gas port and you won't have much trouble. All that said, I prefer the mid-length since it's somewhat more forgiving to less than optimum port sizing and mix and match parts.

October 20, 2014, 08:51 AM	#8
THORN74 Senior Member Join Date: May 30, 2010 Location: Chicago 'Burbs Posts: 543	Should have mentioned in my original post. I already have a midlenght, for some time now. So while I appreciate the other info, I was really just concerned with the theory I proposed. I am well aware of the other advantages, its why I went mid length in the first place.

October 21, 2014, 11:24 PM	#10
btmj Senior Member Join Date: November 1, 2011 Location: Near St. Louis, Missouri Posts: 864	I have both. From a shooting standpoint, the mid=length seems to have a slightly softer recoil... but the gas length does not make nearly as much difference as a heavy barrel... An 16" HBAR barrel adds over a pound to the rifle compared to a 16" M4 profile, and that makes more difference in recoil than the gas length.

October 23, 2014, 07:40 AM	#11
mxsailor803 Senior Member Join Date: February 8, 2010 Location: SC Posts: 1,344	To the OP, in theory, you "should" gain a little FPS. Now thats just in theory. I don't even know if it would even be a big enough difference to worry about. Even with handloads, its next to impossible to get the exact FPS repeatable enough times to accurately measure a difference. Example, you want to do a 5 shot average FPS, now you will need 10 rounds to accurately measure the difference. I'm not saying its impossible to get a exact measurement, just won't be easy or (for me at least) justifiable difference.

October 23, 2014, 09:54 AM	#12
tirod Junior member Join Date: January 21, 2009 Posts: 1,672	With Carbine gas, to be milspec it came only in a 14.5" barrel, and that is a notable subject of discussion from the events in Afghanistan. Many considered ti to have too little effective range in the long distance shooting wars there. So was a 16" barrel. The Army went back to a 20" one per squad, refitted the M14 (the few Clinton didn't have demilled,) and started supporting patrols with crew served weapons. Barrel length has more impact that where the gas is tapped. And even on the same length barrel overall, the gas port size is different. That also effects the results. All said, too many variables to make a comparison when the ammo loading process and rifling itself are more likely to affect the results.