AR question: gas system, rifle vs. mid

[readyme]

I am considering getting my first AR.* One topic that keeps coming up is the length of the gas tube.* I have heard the least reliable over time is the carbine length, so that is not part of my question.
The new "it" thing seems to be mid length systems…but why do people avoid 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?).* One thing I read recently is the USMC has stayed with the 20" barrel while the Army's M4 is shorter (not sure of the length).
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 16" vs 20"?
Your thoughts and opinions are needed.

 

[taliv]

you can't run a rifle length on a 16" barrel (search on "dwell time"). i have rifle length gas on a 17" bbl and a 18" bbl and a 20" bbl.

 

[john5036]

Why do you perceive that rifle lengths are being avoided? A rifle system would be too large for a 16" I would think. I don't know the technical reasoning though. If anything, I just think they are relegated to 2nd and 3rd ARs, with the carbine/mids being practical purchases, like home defense, whereas rifle lengths are being purchased in higher calibers and longer barrels for longer range sport shooting.

 

[Z-Michigan]

Rifle length is softer, smoother and more reliable than midlength, if your barrel (17" or longer) can work with it, as noted.

Most people lately just seem to want 16" or shorter ARs. 20" is long for an AR but is not long in absolute terms, in fact very short, light and handy compared to most other types of rifles. If you think you want a 20" rifle, go for it! If you decide you want a 16", midlength is what you should get. In between those, rifle gas is preferable, but different companies vary in what they put on their 18" barrel models.

 

[Send_It]

16" can be run with a rifle length gas system

Readyme,

Contrary to the last post (sorry Mr Moderator), You CAN run a 16" barrel effectively with a rifle length gas system. You may have to open up the gas port a bit to allow more gas flow into the reciever in order to cycle the gun reliably.

As Taliv mentioned, when you have a rifle length gas system on a 16" barrel, the residence time of the bullet past the gas port (dwell time) is decreased dramatically. This residence time or dwell time serves to build up back pressure in the barrel. This back pressure is what u rely on to force enough gas back into the receiver to properly cycle the action.

In order to get a rile length gas system to run efficiently in a 16" gas tube, there are three things that can be done;
1. The gas port needs to be large enough to make up for the dwell time you've lost. You wont know whether or not to open up your gas port until you shoot your least powerful load. Since gas port sizes varies between manufacturers, if i'm afraid you wont know ahead of time.
2. You can run hot loads exclusively. Hot loads contain more powder, and by extension, build up a higher gas volume and gas pressure in the barrel and may be enough to reliably cycle your action.
3. Attaching a muzzle device. This increases back pressure on the gas port, forcing more gas into the action. The amount of back pressure varies with type of muzzle device.

My first AR was a DPMS LR 308 with a 24" SS bull barrel. After a few trips to the range, i decided that this gun, while accurate and reliable, needed to loose weight so that i can take it with me for hunting, carbine courses etc. I sent the barrel off to ADCO and had it re-profiled to .750" (.938" is a bull diameter) and i also had the barrel cut down and threaded. I have run about 250 rds of everything from low power crappy Russian steel case ammo (Monarch) to hornady superformace (hot loads) to my hand-loads (moderate loads cause I'm cheap) with no problems to speak of. So i know first hand that it can be done. BTW, I DID NOT have to open up the gas port or anything.

Additionally, Knights armaments makes their 16" SR-25 EMC with a rifle length gas system. So it looks like me and knights armaments are some of the only ppl crazy enough to opt for this gas length in their carbines.

I hope this little tid-bit helps. The short answer to your question is that ppl go for midlength gas because they are running 16" carbines and are afraid that their rifles wont cycle reliably with a rifle length gas system.

Send_It!

 

[taliv]

I wasn't implying it's not theoretically possible. Just that no manufacturer I know of sells a 16" or shorter barrel with a rifle length gas port of any diameter.

Also knights gas systems are usually called "intermediate length" because they are shorter than rifle but longer than midlength. The emc is a 308 which is much different than a 223

 

[Send_It]

AR Gas System

Taliv,

Bushmaster makes (or made) a 16" dissipator carbine in .223 Remington. So there's a manufacturer that makes a 16" carbine in a smaller caliber.

In all honesty, the caliber doesn't matter. While a .308 operates at higher pressures and gas volumes, the BCG of a 308 is twice as heavy as that of the .223. That means that the gases from a 308 bullet has to do more work to move the BCG of a 308.

It all comes down to doing a pressure balance. For a 16" AR to run reliably with the rifle length gas system, the gas port needs to be large enough to compensate for the loss of dwell time relative to a longer barrel, the load being shot needs to contain enough powder, and the muzzle device affixed to the barrel also helps some. The gas port on a 16" barrel should be sized so that it will cycle all available commercial loads (except sub-sonic loads)

Regards,

Send_It!

 

[taliv]

Most dissipators are rifle length sights but have a carbine length gas port under the handguards.

 

[Bartholomew Roberts]

You can make a 16" with rifle length gas run; but much like the 10" shorties, the "window" where it runs well is much smaller. Size the port to run well with low-powered .223 Russian stuff and it will be overgassed with 5.56x45. Size it for NATO and it will short-stroke with the Russian stuff.

 

[TonyAngel]

A 16" or shorter barrel with a rifle length gas system can be made to work, but the rifle is going to be finicky about ammunition and won't be very reliable. Besides, there's no reason to even want to go with a rifle length gas system. A 14.5" or 16" barrel with a mid length gas system shoots pretty darned good, bested only by that of a 17 or 18" barrel with a rifle length gas system.

 

[readyme]

Thanks for the info guys.
The gas system depends on the barrel length.
So, it would seem that more people are opting for the shorter barrel length...I would assume that is because people want a more "handy" and fast rifle.

What is the ballistics difference between a 16" barrel and a 20" barrel? If one is to shoot 5.56 rounds through these barrels, would there be an appreciable difference?

 

[kwelz]

Depends on the range. But unless you are shooting way out there I don't think you are going to see much difference in ballistics of hitting power between a 20 and 16 inch gun.

 

[LKB3rd]

The people using mid length gas systems are doing it on 16" barrels. It is my understanding that the carbine length gas system is optimal for the 14.5 " M4, or other military carbines, and since its close, ends up on civilian ar's with 16" barrels. Some people like them with mid length gas systems, and from reports they work well.
The main advantage of a 20" barrel is a longer sight radius. It will have higher muzzle velocity, but I am not sure if that is necessarily an advantage.

 

[kwelz]

The people using mid length gas systems are doing it on 16" barrels. It is my understanding that the carbine length gas system is optimal for the 14.5

Not really.
BCM, DD and Noveske are all producing 14.5 inch Middy guns now. I have one as my training Rig (A Daniel Defense). They have all the benefits of a 16 inch mid length gun without the extra length.

 

[Z-Michigan]

Just that no manufacturer I know of sells a 16" or shorter barrel with a rifle length gas port of any diameter.

That's apparently how Del-Ton has designed their recently introduced dissipator model. I don't know of any others. AFAIK Bushmaster's dissipator had a gas block under the handguards at the normal carbine position.

It may be possible get a 16" rifle gas setup to run with a humongous gas port. Any benefits vs. a properly setup midlength 16" would be small, and you would be taking on a lot of unpredictability and likely unreliability to do it.

 

[Zak Smith]

A friend of mine cut down a 20" to 16" and used that for his 3-Gun rifle for a long time (RLGS).

I've used 17's for years and they run reliably down to the coldest temperatures we've had here, with commercial ammo, no problem.

 

[RP88]

the reason why most people avoid the rifle length is because they aren't very fitting for carbine roles.

A 16" barrel or shorter, however, is.

and a 16" midlength is the better way to go over a 16" carbine. The midlength is a much more natural and proportionately correct system for a 16" barrel. And, even though this does not matter, you can actually mount a bayonet on the midlength's lug properly. You also get a slightly better performance and better sight radius.

But, anything shorter is best done with a carbine length. 14.5" can be done with a midlength as well, but at that length I'd rather just use a carbine length.

 

[68wj]

What is the ballistics difference between a 16" barrel and a 20" barrel? If one is to shoot 5.56 rounds through these barrels, would there be an appreciable difference?

This is from the M855a1 report, but I guess the comparison is still valid (especially since you are not limited on what bullet you can use). M4 (14.5") and M16 (20")

 

[crossrhodes]

Here we go...again

The manufacturers have done a pretty good job of doing the work and engineering on the different gas system lengths and mods to accommodate them. I wouldn't waste my time or money on whipping this dead horse. You have to ask yourself: can you, are you, really that good of a shooter to really have the length of the gas system upset your shooting and mess up your ODDA LOOP. It's nice to know how different lengths work for educational purposes, especially if you are a gunsmith and do that type of work and need to know the effects of mods to barrel and gas tube lengths. I wouldn't get to wrapped around the axle on this and just enjoy the AR you decide on.
BCM, LMT, COLT, KNIGHTS and the rest of the crowd have done the work.

 

[Gary G23]

Not really.
BCM, DD and Noveske are all producing 14.5 inch Middy guns now. I have one as my training Rig (A Daniel Defense). They have all the benefits of a 16 inch mid length gun without the extra length.

I agree with kwelz. I've owned many AR's and strongly believe the 14.5" mid-length is perfect for my uses. The carbine length gas system was actually designed for the 11.5" barrel.

 

[Tirod]

It's all about dwell timing.

Gas tube length is set about 5-7 inches from the muzzle, not how ever long it can be from the chamber. Any gas tube length can be made to work on just about any length barrel, but when optimum functioning is the goal, it's been about 5-7" from the muzzle. That times the opening of the bolt after the bullet leaves the barrel.

One way of looking at it is what most manufacturers put on their barrels - pistol gas on 11" and under, carbine from 11-14.5", midlength on 16", intermediate on 18", rifle on 20". A lot of 16" dissipators run midlength now, if my case of one counts.

Optimize the gas for the barrel and get the best timing, it will cover most of what civilian rounds we shoot thru it. The one problem area has been the cheap low powered import ammo which never meets milspec pressure requirements. Lots of shooters complain it's prone to short stroking, so some makers opened up the port to get it to function. It's a tradeoff, if one of those guns is fed milsurp exclusively, it's overgassed - the problem seems to have surfaced with carbine gas guns - but they are out of the optimum window with a short system and long barrel anyway to get around the BATF's restriction on length, but still sell M4geries.

That's why the trend to midlength, it's not a fashion choice, the better makers decided to just do it right and prevent customers having problems and overloading them with complaints.

Choose the caliber and barrel length for the job you want done, and the gas length generally offered just comes with it. It's not a fashion choice (dissipators are after rifle length iron sights.)

 

[griff383]

I have a 16" barrel rifle length gas system and have never had a problem. I have only put a few select factory loads through it since I reload, but every round I put in it fired and cycled perfectly. I do however have a linear compensator which may help or even be the reason why it works.

 

[otomik]

The carbine length gas system was actually designed for the 11.5" barrel.

Correct sir, the carbine length system used on the m4 originates from the XM177. The m4 uses it because that's some stuff they had in inventory at the time they were developing it. they simply lengthened and profiled the barrel to mount the grenade launcher and bayonet.

i like the 14.5 inch with permanent flash hider and mid-length gas system. i think that will be a popular config. and it just looks right.

 

[Quentin]

...i like the 14.5 inch with permanent flash hider and mid-length gas system. i think that will be a popular config. and it just looks right.

I agree this configuration looks good and is getting more popular but I'd much rather have a 16" midlength (and in fact have two). When it's all said and done there's about 1.1" difference in overall length and with the 16" you don't have to pin the muzzle device due to legal issues (unless you're in a ban state).

 

[mister2]

Very interesting and informative thread. The takeaway is that there is no one answer and it's the combination of application, dwell specs and perhaps use of specialized ammo (IIRC some bullets stabilize better with longer barrels) that determine final configuration.

Good luck O.P.!