How To Choose a New Inner Barrel


So you’re upgrading that airsoft gun and you’re thinking… well I want more accuracy, more range, and more fps… I must need a new inner barrel.  Well, you might be surprised to find out that a new inner barrel could actually hurt you in reaching your goals.  Today we’re going to look at how to keep from going two steps back while trying to get one step ahead of your competition.

Goal Setting

First of all we need to know what the goal is that you are looking for.  Is it an increase in FPS?  Is it improved accuracy?  Is it both?  Is it nothing more than just changing the barrel length due to a change in your outer barrel or appearance of your gun?

Getting Our Bearings

Now let’s think about this. Say you’re at the mall and you’ll trying to find your way to a department store to pick up a gift for someone but you don’t even know what to buy. What’s the first thing you need to do? Well, first things first, let’s figure out where you’re at. See that bright red circle that says, “You Are Here”? That’s what we need to determine. Where are you starting? For this we need to know 3 things.

1. What is the current inside diameter of your inner barrel?

2. What is the current length of your inner barrel?

3. What are you trying to achieve through changing your inner barrel?

Finding Our Destination

Now that we know where “You Are Here” is, we need to figure out where to go.  For that we check out the possible gift list.  What’s on your shopping list (question 3 from above)?  Accuracy?  FPS?  Maybe both?  For the purposes of this example, let us look at both accuracy and FPS.

Frankie’s Prosthetic Shop

Frankie owns a prosthetic store in the mall where he sells feet.  He’s a pretty good guy, and the fact that he has a robotic leg makes him that much cooler.  His business is going so well, that he sells prosthetic feet by the second.  What Frankie has learned is that in order to increase the speed of production he has to streamline the process.  By streamlining the process, he has created a way getting his product to the customer very quickly, but the chance of error has increased.  Okay, if you’re not seeing the metaphor here I’ll make it simple.

In order to increase the speed of the bb (increase the feet per second — FPS),  you have to increase the pressure forcing the bb out.  When it comes to smooth bore barrels there is only one way to do that — decrease the inner diameter (ID) of the inner barrel.   By decreasing the ID of the barrel, less air can escape or float around the bb, and more of it is directed behind it.  It’s like sticking your finger on the end of a water hose.  By covering up a bit of the hose, you’ve increased the speed with which the water comes out.  The problem now lies in trying to keep that bb stable as there is less air around the bb guide the direction of travel.

Waldo’s World of Watches

Waldo also has a store in the mall.  At his store he sells watches. Now we’re not talking about the digital kind, or even the likes of Rolex or Cartier…. those are junk compared to these handcrafted masterpieces.  Each watch is manufactured to very tight specification.  Every detail is looked at.  Each cog of each gear is machined perfectly and polished to be as smooth as possible.  Since everything has to be watched over and guided carefully though the entire manufacturing process, it takes a long while to complete a watch. The end result is that each timepiece is accurate down to the millionth of a second.  Okay, if you’re still not getting it we’ll again, make it simple.

As the bore of the barrel becomes tighter, there is less airflow around the bb.  That airflow is necessary as it directs and maintains the path of the bb’s travel.  The more air found around the bb, the greater the bb’s ability to stay on course to the target.  As we tighten the bore, we decrease that cushion of air.  Now here’s the catch.  There is a fine line in how much slack (or air cushion) we need in order to keep it on track and the point at which we see a drop in accuracy.  How tight of a bore can we get before we see accuracy decrease?


On our way around the mall we’re stopped by a guy who has a stack of pamphlets in his hands.  He is standing there shouting out a message saying,”What you need is sometimes what you already have!  Seek not that which you do not need!”.  Puzzled we ask the old man for his wisdom.  He was more than grateful that someone actually stopped to talk with him that wasn’t mall security for once.  After hours of walking about the mall and talking and hearing the wisdom he had to offer we come across a crowd.  As we make our way through it I loose sight of the wise old man. Arriving on the other side of the crowd the old man was gone…. and so was my wallet.

I learned two things.  First, sometimes people are just more than happy to take your money and run with it.  These people work in one of two industries.  Marketing or pick pocketing.  Many companies would have you believe that the tighter the bore the better the barrel and the better the airsoft gun.  That only holds true if you are concerned with FPS.  Others would have you believe that you must upgrade your barrel or you will be losing out in performance.  The other thing we learned, is to look at what we already have.  Many airsoft guns now come with a 6.04mm inner barrel STOCK.  Of course quality plays a part, but when it comes down to it, the difference in performance between a 6.04 factory barrel and a 6.04 aftermarket barrel is probably less than the deviation created by your ability to shoot and the effect of the environment.

Tammy the Tailor

Realizing that the old man didn’t steal your wallet, but rather a large hole in your pocket was the problem, you decide to stop off at Tammy the Tailor’s shop and get it fixed.  While there, sitting in my boxers (with embarrassing pink hearts on them… don’t ask), I struck up a conversation with Tammy.  Tammy was telling me what it was like to make clothing and how she had done upholstery and worked in other areas of the textile industry before opening up her tailoring shop.  She told me how the materials used to make things played a part in the quality of the final part.  For instance how different types of leather made for different quality saddles, and how a bed sheet made of nylon wasn’t as soft as one made from cotton.  I found this quite interesting.  I was thoroughly engaged in the conversation till I heard her say, “Whoops!”  What I didn’t know is that Tammy is legally blind and she had mixed up my pants with an order for Frankie and cut off one of the legs of my pants.

So what did we learn from Tammy?  Well, for one barrels are made of various types of materials.  Everything from brass, to steel, to aluminum.  The thing to remember is that even within a single metal alloy (such as Aluminum), there are different grades and specs.  So what is the best one for an airsoft inner barrel?  Well, you would need something that won’t change shape with temperature, and isn’t easy to warp.  So as far as that goes, steel is by far the best choice.  The problem here is that it’s also the most expensive.  Brass is really soft, is easily damaged in comparison, but is less expensive.  Then there is Aluminum.  There are so many types of aluminum, but in the end it’s much lighter than steel, and harder than brass.  It truly is a good middle ground.  In the end I don’t know that you’ll be exposing your gun to enough heat or cold to cause the barrel to change shape enough to reduce the consistency beyond the tolerances of an airsoft projectile’s trajectory.  Remember, it’s airsoft not real steel.  Any of these materials should do you just fine as long as you’re using good quality bbs.


Here is what barrel choices come down to.  You have to make a choice do you want more accuracy or more velocity over your stock setup.  If you want more accuracy and you are already running a 6.04 ID inner barrel, then you must chose to decrease the velocity in an attempt to increase the accuracy.  On the other hand, if FPS is what you’re looking for you can just choose to tighten the bore and expect positive results. Most of us are looking for something in the middle.  This means a compromise between speed and accuracy.   Given that barrels often come in sizes from 6.01mm to 6.08mm middle ground of 6.03-6.04mm will yield the best results.

Inner Barrel DiametersPDI created the image to the left.  I have scanned it from their 2008-2009 catalogue.  It depicts what most manufacturers don’t tell you.  While I can’t read most of the advertisement, the picture really speaks for itself.

The way I see it, if you’re running a stock 6.04mm inner barrel, it’s really not worth it to change it out if you’re looking for that middle ground.


2 responses

28 01 2009
New Tech Article « The Festering Wound of the Airsoft World

[…] Link How to Choose an Inner Barrel […]

12 11 2013
DannyFingerbang Deridgablechimney VonRoff

thanks for this. its helped guide me in my quest for suitable upgrades for my rifle. well writen too

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