Carbine vs Mid Length: Ultimate Rifle Gas System Comparison

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If you already made the move up from a pistol to a rifle, no matter if for the firing range or personal protection, you’ve surely gone through the carbine vs mid length dilemma already. Or maybe you’re thinking about buying your first rifle, and you want to make sure you make the best decision when it comes to gas tube lengths for your new acquisition. One thing is certain, you don’t want to go to the shop unprepared and risk letting the seller give you anything other than the best fit for you. And while there are lots of things to take into consideration when getting a new rifle, starting with the gas system length is an excellent approach to determine what model you end up with.

Thus, we’ve decided to take a closer look at the carbine vs mid-length systems, and see what both of these offers and which would be a better choice for you.

Gas System Basics

Before jumping into details on rifle length gas system types and the benefits you get from either carbine vs mid-length, it’s wise to understand the gas system concept altogether. A rifle gas system is one that operates using gas generated by the fired cartridge to create the energy needed for autoloading firearms. This is also called direct impingement. In other words, a part of the energy generated by firing one bullet is harvested through a special system to eject the spent case and load a new bullet. If reading about it doesn’t give you a clear idea of how a rifle gas system works, check out this animation:

how does gas system work in rifle
Image source: Wikipedia

Why Does the Length of the Gas System Matter?

Here’s where the carbine vs mid length gas system discussion begins. As a general rule, a gas system gets bigger when the length of the barrel increases. The size of the gas system used on a rifle is important because it dramatically influences the dwell time. And if you’re not familiar with the firearm slang, “dwell time” refers to the time a bullet spends in the barrel after you press the trigger. You also get an extended dwell time with a longer barrel, meaning there’s a bigger delay between successive shots.

Also, too much barrel length for the gas system could lead to too much gas flowing into the receiver, causing excessive recoil and even increasing the rifle’s wear. On the other hand, if the barrel is too short, there’s a chance for not enough gas to flow into the system, and the rifle can jam or not cycle properly.

What is the Correct Gas System Length for my Barrel?

To find an answer for the never-ending carbine vs mid length question, you need to consider two things – barrel length and port distance. We put together a table to help you visualize standard gas system sizes for various barrel lengths.

Gas System Barrel Length Port Distance
Pistol Under 10” 4”
Carbine 10 to 18” 7”
Mid Length Gas System 14 to 20” 9”
Rifle Over 20” 12”

As you can see, when deciding between carbine vs mid length (since that’s our main focus), you should be aware that you’re going to deal with a 2″ difference in port distance in favor of mid-length. Again, these figures offer the best performance and are used by most manufacturers out there, like the Colt AR-15 for example. The standard barrel length of the AR-15 is 20″. However, the weapon is also available with 16″ carbine barrels. The same goes for the AR-15 lower parts kit that can be customized to the user’s preferences.

Other Factors Affecting the Gas System Beyond Length

No discussion about carbine vs mid length gas systems and barrels won’t be complete without considering other factors that can influence their performance. Here’s what you need to know:

  • Adjustable gas systems: Some systems come with the possibility of adjusting them to find the perfect settings for you. You can control the gas flow to such an extent that it turns a rifle into a single shot.
  • Ammo used: The amount of powder within the cartridge and the bullet’s overall weight can also influence how a gas system operates. Shooting a heavy bullet will cause more dwell time and a lower quantity of powder may not generate enough gas for a cycle.
  • Buffer weight: Another thing to consider when deciding between mid length vs carbine systems is the buffer’s weight. If you go for a heavier buffer, you’ll need more gas pressure for a cycle and the other way around.

Carbine vs Mid Length Gas System Main Differences

Since you reached this part of the article, you’ve undoubtedly spotted a difference between carbine and mid length systems every now and then. However, to keep our word and deliver an exhaustive review that will help you choose either the carbine or mid length gas system, we decided to highlight the main ones for easy comparison.

Gas Port Pressure

To compare gas port pressure, we will use the AR-15 architecture since it’s prevalent. Without tackling the AR-10 vs AR-15 discussion that is so common among rifle owners, we invite you to observe the approximately 5,000 PSI pressure difference between mid-length and carbine in the table below. This translates into a better bolt velocity for mid-length models resulting from the lower pressure.

Gas System Ammunition Chamber Pressure Max Port Pressure
Carbine NATO M855 33,000 PSI 26,000 PSI
Mid Length NATO M855 27,000 PSI 21,000 PSI

Dwell Time

As discussed earlier, a mid-length system will also improve the dwell time on a standard 16″ barrel compared to a carbine. Not just that, but it will also give you less recoil and possibly expand the lifespan of your rifle. Overgassing ( getting too much pressure) will also lead to excess debris and gas, making the gun dirtier in the long run.

Gas System Barrel – Gas System Dwell Length
Mid-length 16” – 9.5” 6.5”
Commercial Carbine 16” – 7.5” 8.5”
Mil-spec Carbine 14.5” – 7.5” 7.0”

Handguard Length

Another difference you need to be aware of when comparing the carbine length gas system to a mid-length one, is the available handguard size. Sure, this only applies to models with handguards that do not go past the gas block. However, having an extra 2″ of handguard in the case of mid-length can improve your shooting technique and overall grip. This is especially important if you fit lasers, lights, or other gadgets in on your weapon.


To settle the mid-length vs carbine dilemma once and for all, we share with you the results of heavily testing of the mid-length and carbine gas systems by the NAVSEA Warfare Centers. Captain Mark Oesterreich, USN Commanding Office of the NSWC Crane and Dr. Brett Seidle, SES Technical Director NSWC Crane signed the report that came up with the following results after 12,600 rounds:

  • 30 malfunctions for mid-length gas systems
  • 65 malfunctions with carbine length gas systems
  • A slight decrease in the cyclic rate for automatic fire when using the mid-length system

Final Words

After going through all the data and arguments presented above, it’s time to draw the line and conclude the carbine vs mid length gas system choice once and for all. At the end of the day, it’s a matter of personal preference and even looks for some users. However, you can’t ignore the fewer malfunctions over almost 13,000 rounds fired for the mid-length.

Also, the lighter recoil and fewer debris make compelling arguments for choosing mid length over carbine systems. When you also add the extra 2″ of the handguard to the mix, it’s tough to make a case in favor of the carbine length gas system. The only upside comes from an improved firing cycle but with the cost of higher recoil and wear on the weapon.

So with that said, do you have any experience with mid-length vs carbine gas systems? What’s the perfect gas system/barrel length combination for you? Feel free to share your thoughts and experience in the comments below!. After all, there’s nothing like hearing the opinion of a fellow gun enthusiast.