Carbine vs Mid vs Rifle Gas System

I've heard people say that the Mid length gas system is better than the carbine length. But, does anyone here have the straight skinny on WHY one is better than the other?

I have rifles with carbine, mid and rifle length systems. None have ever failed to cycle or feed properly. I just don't see why there's a dust up over this.

I'm glad you asked....

I was perusing this topic just few days ago and wasn't aware of any better/worse.

It comes down to recoil impulse, dwell time and wear and tear on parts based on the barrel length and which gas system you are using.

Really it is just a preference IMHO. As a general rule of thumb people most often recommend carbine gas on 14.5" rifles, mid length on 16" and rifle 18" and up. Some manufacturers like BCM use mid length in 14.5" and 16" guns.

As a general rule the longer the gas system the lower the recoil impulse. Many believe that the carbine gas is needed for reliability in a 14.5" gun but you get a lighter recoil impulse and less wear and tear on a mid length without sacrificing reliability at 16".

Instead of retyping the wheel here are 2 decent explanations.

Http://www.03designgroup.com/technotes/carbine-vs-mid-length-gas-system

Shooting Illustrated | AR Gas-System Lengths Explained

Rastoff said:

I've heard people say that the Mid length gas system is better than the carbine length. But, does anyone here have the straight skinny on WHY one is better than the other?

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They each have their own application, so the idea that one is necessarily better than the other is the wrong way to look at it.

Using more gas than is required to reliably operate the rifle will result in greater felt recoil and perhaps greater wear on the rifle. Using less gas than is required to reliably operate the rifle will result in an unreliable rifle. So... the idea is to try to get close to the correct gas system for the intended purpose.

The links above do a good job of explaining the different systems.
With 16in barrels, most manufactures sell their rifles with carbine gas systems. Typically they are over-gassed. However, they reliably work with the huge array of off the shelf ammo and will more reliably continue to work with a poorly maintained rifle. That's good for shooter and manufacturer. The downside is that there is more felt recoil and perhaps slightly more wear. When you start reducing the gas with a mid-length system or shortening the barrel then you move in the opposite direction... less felt recoil but less margin for reliable operation. With a properly maintained rifle and ammo, mid-length systems on a 16in barrel are generally considered "better".

Like the author in Shooting Illustrated, I too am a big fan of adjustable gas blocks. I use a Syrac and can tune the gas system to whatever I want. The adjustable gas block on my carbine gas system made a huge difference in felt recoil and keeping the muzzle down between shots.

I also think a lot of the "debate" comes from the fact that Colt/FN use a carbine length gas system on the 14.5" M4s they sell the govt. We get the exact same gas system but traditionally the civilian version the 6920 has a 16" barrel.

The felt recoil is different with the 16" barrel but for Colt it made sense to use the exact same parts minus the barrel for economy of scale and the fact people get hung up on milspec.

So you ended up with more gas than necessary in a 16" AR15. This then became the standard in the commercial market until other companies started to deviate.

I personally prefer the mid-length gas because even though the recoil impulse of a carbine system in a 16" barrel is not heavy by any means with a mid length gas system the operation is smoother with less muzzle rise which allows faster follow up shots. For me at least.

Here is an nice propaganda video from BCM... LOL

[ame]https://www.youtube.com/watch?v=yU8ryJN9E5s[/ame]

The only other thing I would add is that the shorter the gas system, the faster the gas port erodes, generally speaking. Not that it's a huge issue usually. By the time erosion becomes a problem, the barrel is pretty much used up as well.

One nice thing about the longer gas systems is that there is more hand guard to grip. Personally, I like mid-length rifles best because of the reduced port erosion and the longer hand guard. There is a bit of a softer recoil pulse but we are talking about a 5.56 rifle, that really doesn't have much of a recoil pulse anyway.

sgtsandman said:

There is a bit of a softer recoil pulse but we are talking about a 5.56 rifle, that really doesn't have much of a recoil pulse anyway.

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It's true that the 5.56 doesn't have a great amount of recoil. And for guys who shoot the rifle supported slow firing trying to make small groups on paper recoil is of little consequence. However, shooting offhand is another matter. Small reductions in recoil translate to significant reduction in muzzle rise affecting speed and follow up shots.

ChattanoogaPhil said:

It's true that the 5.56 doesn't have a great amount of recoil. And for guys who shoot with the rifle supported trying to make small groups on paper recoil is of little consequence. However, shooting offhand is another matter. Small reductions in recoil translate to significant reduction in muzzle rise affecting speed and follow up shots.

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Add movement to that. If you range allows it set a target at 50 yards. Load a mag and walk forward shooting 15 rounds. Then walk backwards firing the next 15. You will quickly notice that any movement over your body movement will be exaggerated. The less your muzzle rises the more accurate and faster you are on target.

ChattanoogaPhil said:

It's true that the 5.56 doesn't have a great amount of recoil. And for guys who shoot the rifle supported slow firing trying to make small groups on paper recoil is of little consequence. However, shooting offhand is another matter. Small reductions in recoil translate to significant reduction in muzzle rise affecting speed and follow up shots.

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That's what muzzle brakes are for.


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Several people have touched on it, but the closer to the chamber the gas is tapped from the barrel (and thus the shorter its travel down the gas tube) the hotter (and thus more errosive) it is going to be when it reaches the gas key and bolt carrier.

In general, the "cooler" the gas is when it is tapped and arrives back at the bolt, the better it is for the longevity of all the parts involved. Of course, as other have pointed out, this may more of a accademic concern since most rifles will likely to outlast their owners.

Despite changes in the port size, the gas presssure in the carrier is about 50% higher in the carbine length gas system than it is in the rifle length. During development of the carbine, it was determined that a different extractor spring was necessary to achieve similar reliability and service life. While it works, the stresses are naturally higher. If one goes with the mid-length gas system, the pressures are lower and reliability is enhanced.

In a test involving low cost range ammo, testers discovered the mid-length system to function more reliably with ammo of less than stellar quality. It should do better in bad conditions also. When I broke down and actually bought my own, it had a mid-length gas system.

You can in fact run a full length gas system with a 16 inch barrel. However, I'd expect that when conditions got marginal-or worse-reliability would suffer.

US military rifles have virtually always had non-adjustable gas systems. One less thing for the troops to screw with/up.

BTW, the way the Shooting Illustrated guy explains how the "gas expansion system" works isn't correct. The delay in unlocking really isn't because "the gas is pushing against the back of the bolt". The delay is caused by the change in gas velocity/pressure when the gas enters the greater volume of the carrier itself. It takes a small, but finite time, for the pressure in the carrier to build to the point where the carrier begins to move and unlock the bolt.

Myth to the contrary, the gas system never sees the maximum pressure in the chamber at peak. As the bullet (and gas) moves down the bore the volume the gas has to fill gets larger and the pressure drops.

WR Moore said:

You can in fact run a full length gas system with a 16 inch barrel. However, I'd expect that when conditions got marginal-or worse-reliability would suffer..

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Due to (relatively) earlier exit of slug, from muzzle, and curtailed pressure pulse?

Good thread, anything that helps a shooter maintain his point of aim is worth looking into. Hitting what you aim at is what it is all about.

Steve912 said:

Due to (relatively) earlier exit of slug, from muzzle, and curtailed pressure pulse?

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It's not really a pulse. The size of the gas port determines the rate of gas flow into the gas system. Once the gas enters the gas tube, the increasing volume it has to fill lowers the pressure and this happens again when the gas flow enters the bolt carrier.

The shorter distance to the muzzle reduces the volume and pressure of gas in the bore that maintains the flow into the system.

It's very like putting a hole in a garden hose very close to the end of the hose and putting a short burst of pressure into the hose. You get a longer flow if the hole is further back from the end of the hose.

It's been a long time since Fluid Dynamics, can't do much better. If we're being really technical, the flowing gas is actually compressing the air within the gas system. Which means that the actual total volume of the gas system is the entire length and the unlocking delay is the time necessary to pressurize the whole thing to whatever's needed to start moving the carrier.

FWIW, the M4 gas system had it's origin in the XM177 which had a 10.5 inch barrel. And used a flash hider that made overall length barrel length about 16 inches. I believe they anticipated use of a suppressor.

WR Moore said:

It's not really a pulse. The size of the gas port determines the rate of gas flow into the gas system. Once the gas enters the gas tube, the increasing volume it has to fill lowers the pressure and this happens again when the gas flow enters the bolt carrier.

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Right...I was visualizing the pressure dropping throughout system as soon as slug exits muzzle/flash suppressor, with that drop happening sooner on barrel with 'less space' between port & muzzle--such as a "full length" gas system on a 16" bbl.

The pressure has to exist long enough to overcome the bolt carrier inertia/friction and buffer spring resistance and accelerate the carrier sufficiently to unlock bolt and sling back far enough to accomplish a load & lock sequence.

Just wondered what you saw in a full length gas system on a 16" that could result in reduced reliability, compared to carbine length gas system...

sgtsandman said:

That's what muzzle brakes are for.

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Not so much as a fix for an over-gassed system but some brakes do work remarkably well to reduce muzzle rise. It's a subject that's rarely visited. We should remedy that. Let's... (new thread)

ChattanoogaPhil said:

Not so much as a fix for an over-gassed system but some brakes do work remarkably well to reduce muzzle rise. It's a subject that's rarely visited. We should remedy that. Let's... (new thread)

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This is true. Over clocked BCGs typically aren't a good thing for the rest of the rifle in the long run. Especially if the gas key is smacking into the receiver.


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Years back, before the mid-length craze really caught on, I got a Spikes middie full upper from AIM, largely because it looked cool, they were offering a great deal, and I vaguely recalled reading that they were somehow a design improvement.....and i had the bucks handy.
Soon after I picked up a built Spikes lower, and had me a new mid-length rifle.
I always felt that rifle was somehow just a bit more pleasant to fire than any carbine-length rifle I'd owned. I hadn't expected there to be any discernable difference at all, but my sense of it was that it had less of that harsh "CRACK!" that AR's have, in comparison to the big, more fun "BOOM" of an AK47.
Of course, the difference would be nearly microscopic, and my first inclination would be to suspect the difference was psychological and only in my head, except for the fact that I never expected any discernable difference.
Anyway, that's what I came away with, that the middies had somewhat more of a cushioned AK-esque "boom" versus the snappy,crisp report and feel of a typical AR.

ChattanoogaPhil said:

They each have their own application, so the idea that one is necessarily better than the other is the wrong way to look at it.

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I agree. It's like a dog chasing its tail, what you going to do once you finely catch it ? Fish or cut bait ?

I think that's what I enjoy the most out of the MSR end of the hobby. Taking a good lower cost low tier AR and fine tuning it to run with my higher end AR's. Adjustable gas blocks, reloading to match bullet weight to twist rates and barrel lengths, different barrel weights and gas systems. They all have their own fine nitch, it's just a matter of fine tuning it to make it work and be reliable. The higher end are "mostly good to go, but you pay for it too. Myself, I prefer mid to rifle length with heavier longer barrels.

Many new people I've found entering the hobby want a cheap AR that shoots cheap dirty ammo and don't want to fuss around with a lot of maintenance. You know the ones, they tape over the check engine light on their car, spend half the summer trying to get their lawn mower to run and the snowblower has been sitting under the bench for 3 years.