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It's widely known that changing inner barrel length and changing inner barrel diameter change the airsoft BB velocity. Physics tells us that. But exactly how much does it change by? It's anybody's guess. All sorts of numbers are posted on Internet forums and discussions. Some people claim it's negligible. Some people claim it's significant. What is it really?
In this article, we will explore this topic with real quantifiable results. We will use scientific method of observation and analysis to determine exactly how much these changes make. We will provide full disclosure to how the test is conducted and what equipment are used. Does changing inner barrel makes that much difference? Let's find out.
For this test, we used the Shark Raptor. We built this gun in the "Build a Science Fiction Airsoft Gun" article. You can read all about the specs of this gun in that article.
We used three inner barrels in this test: 1) Angel Custom G1 Stainless Steel 6.01mm AEG Inner Barrel (110mm); 2) APS stainless steel 6.04mm diameter 175mm length inner barrel from ASR106; 3) APS stainless steel 6.04mm diameter 275mm length from ASR107. The stock APS metal hop-up unit has been adjusted to be accurate and precise out to over 30 feet in an outdoor environment. It has a stock plastic H-hop spacer. The adjustment is accomplished with 0.2g BB's. We will replace the inner barrels without touching the hop-up adjustment, which is very tight. We used Evike Professional Grade 6mm Airsoft BB (0.2g) in this test.
Our chronograph is the Chrono Connect Mobile Lite Android app running on Motorola XOOM tablet. The measurements from this set-up is quite precise compared to real chronograph used on the equipment in this article. The set-up for this app is depicted in great detail in our review for this app. You can read that article to see how the range is set-up for this test. All of these equipment are reviewed in-depth on this site. If you are interested in their specs, please do a quick search on these equipment.
The controlled environment is a dim garage to eliminate external atmosphere factors. The target trap is located 20 feet from the shooter. The shooter will hand-hold the shots. 3-rounds will be fired into the target trap in semi-automatic mode. Target accuracy is not an issue in this article. Therefore, as long as the BB hit the paper target trap, it is sufficient. The shooter is free to take as much time as possible (within reason) to make accurate shots. A red dot sight will be used. The red dot brightness is set to 1 in order to keep the MOA as small as possible, but highly visible this dim environment. But since we are not talking about target accuracy, so the red dot sight is purely for the ease of hitting the paper target, not for the accuracy.
In this test, we tested the inner barrels in this order: 1) Angel Custom G1 Stainless Steel 6.01mm AEG Inner Barrel (110mm); 2) APS stainless steel 6.04mm diameter 175mm length inner barrel from ASR106; 3) APS stainless steel 6.04mm diameter 275mm length from ASR107. In order to be sure the test is controlled, we went back to test the 110mm inner barrel again at the end. The result was still the same. Therefore, we believe the data is accurate.
This was the first test. This set-up had been measured at a local indoor airsoft field recently, so we know the data we got below is good.
This was the second test. This set-up had been measured at a local indoor airsoft field recently, so we know the data below is good.
This was the third test. This set-up had never been measured before anywhere. However, our previous test showed good data and our final controlled test (fourth test) showed good data, therefore, we were highly confidently in the data shown below.
Fact 1: When the inner barrel diameter is the same (6.04mm), changing the length by 100mm causes the velocity to change by 28 FPS.
Fact 2: Changing the diameter by 0.03mm and the length by 65mm causes the velocity to change by 47.3 FPS.
Fact 3: Changing the diameter by 0.03mm and the length by 165mm causes the velocity to change by 75.3 FPS.
Let's start by extrapolating the velocity difference of a 6.04mm diameter 110mm length inner barrel using one relationship:
xFPS 65mm ----- = ----- 28FPS 100mm xFPS 65mm (28FPS) = ----- 100mm xFPS = 18.2FPS
The result above shows that out of the 47.3 FPS difference, 18.2 FPS is from the change in length. In this extrapolation, the 0.03mm difference results in 29.1 FPS change.
Then let's extrapolate the velocity difference of a 6.04mm diameter 110mm length inner barrel using the other relationship:
xFPS 65mm ----- = ----- 28FPS 100mm xFPS 165mm (28FPS) = ----- 100mm xFPS = 46.2FPS
The result above shows that out of the 75.3 FPS difference, 46.2 FPS is from the change in length. In this extrapolation, the 0.03mm difference results in 29.1 FPS change.
Wow. In both extrapolation, the 0.03mm inner barrel diameter difference causes the exact same velocity change. It's either the diameter and length changes produce absolute linear results or I did something wrong in this analysis. If the former is the case, then we can accurately determine potential velocity (the velocity of the inner barrel you want to buy) based purely on one observational data (the velocity of the inner barrel you already have).
I can get more results and perform more in-depth analysis if I have more inner barrels. If anyone want to donate some inner barrels, please let me know.