View Full Version : AR gas system: mid or rifle? (16" or 20")
I am considering getting my first AR. One topic that keeps coming up is the length of the gas tube (therefor the length of the barrel). I am not interested in the carbine length gas system, just the mid and rifle length.
Everywhere I look, everyone is talking about their mid length systems…are people avoiding the rifle length? I would think the longer gas tube would be easier on the BCG and internals (lower pressure?) and the longer barrel would increase the bullet velocity (and accuracy?). The USMC has stayed with the 20" barrel while the Army's M4 is shorter (not sure of the length)... what are the pros and cons of the 20" barrel?
The only reason I can come up with not having the rifle length is the added length and weight…but really isn't the difference only a few inches and ounces?
Your thoughts and opinions are appreciated.
Thanks.
Here it is short and sweet. A mid length allows for a compact package and reliable operation with less abuse than tha carbine gas system.
Barrel length does not equate to accuracy. Barrel length impacts muzzle velocity which will affect it's overall ballisitics especially at longer ranges.
People are avoiding the rifle lengths due to the fact that they want a more compact package.
I am considering getting my first AR. One topic that keeps coming up is the length of the gas tube (therefor the length of the barrel). I am not interested in the carbine length gas system, just the mid and rifle length.
Everywhere I look, everyone is talking about their mid length systems…are people avoiding the rifle length? I would think the longer gas tube would be easier on the BCG and internals (lower pressure?) and the longer barrel would increase the bullet velocity (and accuracy?). The USMC has stayed with the 20" barrel while the Army's M4 is shorter (not sure of the length)... what are the pros and cons of the 20" barrel?
The only reason I can come up with not having the rifle length is the added length and weight…but really isn't the difference only a few inches and ounces?
Your thoughts and opinions are appreciated.
Thanks.
People are avoiding the rifle lengths due to the fact that they want a more compact package.
Sounds reasonable. So what is one 'giving up' by going with the mid length? Ballistic performance?... but how much?
Are they two both equally reliable?
The only reason I can come up with not having the rifle length is the added length and weight…but really isn't the difference only a few inches and ounces?
Differences is weight between the two depends on the barrel's profile as well. Obviously a heavy 20" will be much heavier than a lightweight/A1/pencil profile 14.5". The balance between longer and shorter barels is more apparent than the weight difference IMO.
The shortest barrel I would recommend to anyone just buying their first AR15 is a 16". The pain of having to deal with a pinned muzzle device isn't worth the 1.5" that you would lose going to a 14.5". You can expirement with different devices (rails, muzzle, fsb/LP gas block etc) much easier on a 16" as opposed to a pinned and welded 14.5".
As to realibility, there are 16" mid-length guns that are every bit as realible as rifle systems, considering (both) came from a good manufacturer. That being said, the shortest gas system I would consider for a 16" is a mid-length.
Intermediate (11.5") gas systems are slightly longer, but are usually only found as of right now, on higher end custom-ish barrels. KAC employs a similar system in the SR15, which many have said is one of the best shooting guns on the market.
Don't get too wrapped up in the quest and idea people have that they will use parts to make a rifle recoil like a staple gun. It isn't going to happen. I wouldn't run on the ragged edge of relability for the sake of maybe a 5% recoil reduction.
The ballistic difference really becomes more significant past 300 yards. Plenty of people have been killed by 14.5" guns. Be realistic about what you are going to be doing and at what ranges you will be doing it.
Only you can decide: http://ammo.ar15.com/ammo/
Sounds reasonable. So what is one 'giving up' by going with the mid length? Ballistic performance?... but how much?
Are they two both equally reliable?
100 fps per 2 inches give or take. Terminal ballistics depends on what you are trying to hit. 20" is just maximizing the potential velocity of a 5.56 round. It's easier in general just to use something bigger beyond a certain range.
jonconsiglio
03-19-11, 11:36
I am considering getting my first AR. One topic that keeps coming up is the length of the gas tube (therefor the length of the barrel). I am not interested in the carbine length gas system, just the mid and rifle length.
Everywhere I look, everyone is talking about their mid length systems…are people avoiding the rifle length? I would think the longer gas tube would be easier on the BCG and internals (lower pressure?) and the longer barrel would increase the bullet velocity (and accuracy?). The USMC has stayed with the 20" barrel while the Army's M4 is shorter (not sure of the length)... what are the pros and cons of the 20" barrel?
The only reason I can come up with not having the rifle length is the added length and weight…but really isn't the difference only a few inches and ounces?
Your thoughts and opinions are appreciated.
Thanks.
Do you plan on killing anything out past a few hundred yards that you'd be concerned wouldn't die with 200 fps less velocity?
If the answer is no, there's no advantage to the 20" gun other than slightly less arc, which is neither here nor there when you know what your gun shoots. Get a good middy 16" and learn it well. Quality of the barrel determines accuracy, not length.
Do you plan on killing anything out past a few hundred yards that you'd be concerned wouldn't die with 200 fps less velocity?
If the answer is no, there's no advantage to the 20" gun other than slightly less arc, which is neither here nor there when you know what your gun shoots. Get a good middy 16" and learn it well. Quality of the barrel determines accuracy, not length.
Well said. The only other reason is if you want to do a US mil A2/4 or Canadian C7 (IIRC the model #s) with adjustable stock. Otherwise buy both 16 and 20, then see which one ends up in the safe and which one gets picked to shoot. Unless you're a iron sight radius guy, the 16 is the answer.
Belmont31R
03-19-11, 11:56
The USMC is increasingly using more and more M4's even down into the lower ranks. My best friend has used an M4 in 2 different units. Once in Iraq, and once in Afghanistan. Im not sure if they'll completely get rid of the A4 but I see tons of M4's being used in pics I see. Of course in the good ole Army I got an M16A4.
The prior posters are correct. Accuracy has little to do with barrel length.
As far as the lost of velocity going to a shorter barrel its not something Im going to lose any sleep over. If you're using the gun for HD you'd still be well within the fragmentation ranges of even SBR's.
thopkins22
03-19-11, 12:03
As to realibility, there are 16" mid-length guns that are every bit as realible as rifle systems, considering (both) came from a good manufacturer. That being said, the shortest gas system I would consider for a 16" is a mid-length.
I'm not sure I can agree with this. Is mid-length gas on a 16" barrel better than carbine? Sure. But carbine length gas systems from any manufacturer that I would recommend will serve anyone just fine.
Colt, LMT, DD, BCM, and others put carbine length gas systems on 16" barrels. They're exceptionally reliable.
I'm not sure I can agree with this. Is mid-length gas on a 16" barrel better than carbine? Sure. But carbine length gas systems from any manufacturer that I would recommend will serve anyone just fine.
Colt, LMT, DD, BCM, and others put carbine length gas systems on 16" barrels. They're exceptionally reliable.
I agree. But, taking the time to make a decision and researching the choices, why wouldn't you shoose the system that operates at lower pressures and, subjectively, has lower recoil?
If all you can find/afford is a 16" carbine gas, rest assured it will be a quality weapon that will last for years, provided it came from a good manufacturer. But again, if you have the choice, it seems like an easy one to make between 16" carbine and 16" mid-length. Outside of availibility, I can think of no reason to run a 16" carbine gas.
Realibility is much more than just gas system. It is only one componet of a machine which must work with all the other parts in the system.
I agree. But, taking the time to make a decision and researching the choices, why wouldn't you shoose the system that operates at lower pressures and, subjectively, has lower recoil?
If all you can find/afford is a 16" carbine gas, rest assured it will be a quality weapon that will last for years, provided it came from a good manufacturer. But again, if you have the choice, it seems like an easy one to make between 16" carbine and 16" mid-length. Outside of availibility, I can think of no reason to run a 16" carbine gas.
Realibility is much more than just gas system. It is only one componet of a machine which must work with all the other parts in the system.
Rifle length system on 16" would be better if you want to go to extremes, and yet most people wouldn't select that option if offered (doesnt bushmaster make something like that?). The difference is just not there for some people.
Personally I cant even tell 5.56 has recoil, some muzzle flip yeah, but not recoil.
warriorsociologist
03-19-11, 17:56
Rifle length system on 16" would be better if you want to go to extremes, and yet most people wouldn't select that option if offered (doesnt bushmaster make something like that?).
Yes, but "dissapator" set-ups usually have larger gas ports to enable enough gas to cycle through before the bullet exits the barrel (not much barrel legngth out in front of the gas port on a 16" barrel w/ rifle-length gas). Yes, it can be done -- it's just not my cup of tea. A 14.5" w/ middy gas OTOH.... ;)
I don't think a 16" with a rifle gas system is a viable solution.
You can buy a 16" with an intermediate (11.5") gas system, which give you the same barrel/gas ratio as a 14.5" with a mid.
Kind of, the problem with your ratio idea is that it's a bit too simple.
There is less pressure at the gas port on the longer gas system length, AND it's dropping off faster given an equal length from the gas port to the muzzle.
The rifle gas 16" is not reliable, the "dissipator" uses a carbine gas system (a few use mid now) and a front sight at the rifle position that is not part of the gas system.
The difference in a tight space between an A2 and M4 is huge. The ability to hit targets at range is not so big. Quality defensive ammo is effective out of a 16" well beyond any range you could likely justify for HD/SD.
Robb Jensen
03-19-11, 23:52
Kind of, the problem with your ratio idea is that it's a bit too simple.
There is less pressure at the gas port on the longer gas system length, AND it's dropping off faster given an equal length from the gas port to the muzzle.
The rifle gas 16" is not reliable, the "dissipator" uses a carbine gas system (a few use mid now) and a front sight at the rifle position that is not part of the gas system.
The difference in a tight space between an A2 and M4 is huge. The ability to hit targets at range is not so big. Quality defensive ammo is effective out of a 16" well beyond any range you could likely justify for HD/SD.
As always good info from ToddK.
And with the defensive data on the 16", most will be using an optic and most aren't capable of the 1-4MOA accuracy at 100yds of the dot within that non-magnified optic such as EOTechs or Aimpoints. Many here will beat their chests like gorillas thinking that they are capable of that kind on accuracy but at most about 1-2% of shooters can actually achieve it. And probably much more likely 1/4 of 1% of those could do it when bengi shot at, or while wounded or bleeding or in the mud, rainstorm, sandstorm or rain or snow or in high wind or fog.
Kind of, the problem with your ratio idea is that it's a bit too simple.
Agreed. It was stated with simplicity in mind. ;)
I've never seen actual numbers for the intermediate gas and 16" combo which were tested with burst disks or the like. It would stand to reason that the pressures involved would be slightly lower than a mid, but slightly higher than a rifle. So far I've only run mine with an A5 buffer system, which worked perfectly. When I attempted to use the A5H3 buffer, I had one fail to lock, always using PMAGS of course.
I have a standard carbine RE on the way, and I'm going to test it with all of the various carbine buffers.
So far, it has been great.
I still remember when everyone was saying a 14.5" mid wouldn't be reliable.
warriorsociologist
03-20-11, 00:14
The rifle gas 16" is not reliable, the "dissipator" uses a carbine gas system (a few use mid now) and a front sight at the rifle position that is not part of the gas system.
True -- of the BM model. I remember when a number of gunsmiths/plumbers were making "dissy" set-ups a few years ago by cutting down 20" barrels. I almost bought one. I'm not surprised that idea fell out of favor.
The last chart on this page, at the bottom, I know it's chamber pressure/dwell time but similar to the discussion here?
http://www.ar15barrels.com/prod/operation.shtml
17-18 inches works on a rifle barrel. 16 works well with hotter ammo and in tee shirt weather.
Man, did I ever open up a technical can of worms....I had no idea what I was getting into.
My question was answered pretty early one when I found that the 4" barrel difference is worth about a 200 FPS drop.
THANKS GUYS.
You can go to reloading forum now and ask for the best loading :p
warriorsociologist
03-20-11, 06:51
The last chart on this page, at the bottom, I know it's chamber pressure/dwell time but similar to the discussion here?
http://www.ar15barrels.com/prod/operation.shtml
Yep. That's a big part of what many are talking about. Nice find.
The last chart on this page, at the bottom, I know it's chamber pressure/dwell time but similar to the discussion here?
http://www.ar15barrels.com/prod/operation.shtml
You beat me to it ZRH...
The column that is missing is the impulse, which is average pressure x time.
http://ar15barrels.com/tech/pressure-time.gif
from the chart above, it looks like middy and carbine GS at 16" are exactly the same. At least until it passes the gas port. Is the lighter pressures argument debunked? Am I reading it wrong?
What appeals to me most, is the slight delay in extraction that the middy GS offers, allowing the shell to begin to return to its original size. Unlike the carbine which extracts the shell while its still expanded.
Mr. Goodtimes
03-20-11, 09:54
Doesn't the Sully SLR15 use a 16in barrel and rifle gas system or is id a dissy setup as well?
from the chart above, it looks like middy and carbine GS at 16" are exactly the same. At least until it passes the gas port. Is the lighter pressures argument debunked? Am I reading it wrong?
What appeals to me most, is the slight delay in extraction that the middy GS offers, allowing the shell to begin to return to its original size. Unlike the carbine which extracts the shell while its still expanded.
The overall pressure curve is dictated by the cartridge alone.
The gas system/ barrel length combination determines what portion of the curve gets used to cycle the action.
The same pressure you see is the muzzle pressure, just before the bullet exits the 16". They are both 16" and should be the same.
The row under the gas system headers shows the port pressure, just after the bullet passes the gas port.
The average pressure is the simple mathematical average of the port and muzzle pressures.
from the chart above, it looks like middy and carbine GS at 16" are exactly the same. At least until it passes the gas port. Is the lighter pressures argument debunked? Am I reading it wrong?
What appeals to me most, is the slight delay in extraction that the middy GS offers, allowing the shell to begin to return to its original size. Unlike the carbine which extracts the shell while its still expanded.
you wanna be looking at the gas port pressure (top of each row) not the muzzle pressure.
32900 for carbine
26507 for a middy...
definitely significant...
Oh gotcha. That makes LOTS more sense.
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