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For a long time now techs in the airsoft community have been asking what the perfect barrel length is that gives the most energy for a given bb weight. Some advantages of this are that your gearbox will be under less stress as it will be easier to reach your desired fps rate with a smaller spring. So the gearbox should last longer. Since the gearbox is not working as hard, the gears and the motor are under less pressure and therefore make less squelching noise. The gearbox should also be able to cycle faster since the motor is not having to work as hard. Most of the time, the rifle should also be more accurate. The reason for this is that the quicker a bb can get to its target, the less that outside factors such as the wind will effect the trajectory. Also, since all the cylinder volume is used up, there should be less sound exiting the inner barrel as there is no extra air pushing the sound from the piston slamming into the cylinder head out of the barrel. So your gearbox should last longer, cycle fast and be quieter. Your rifle should also be more accurate and the muzzle volume will be less.

I did an experiment a while ago that answered this big question. In the experiment I used a rifle that had a full V2/V3 cylinder as I wanted to get the maximum amount of efficiency from the rifle. I also used a 550mm barrel (6.03mm diameter) and then fired every bb weight though the barrel with the bb's trajectory traveling straight as possible. I then recorded the data. After doing this, I cut the barrel down by 10mm. Then I crowned the end of the barrel and cleaned it. I then tested the energy rates again of all the bb weights with the trajectory travelling straight and recorded them. I kept cutting the barrel down by 10mm and retesting again. Eventually, after a very long time of repeating the test, the barrel had been cut down that much that there was only a couple of millimetres sticking out of the hopup chamber.

In this experiment I chose to use an M4 rifle as it has the most common cylinder type and it seems to be the most common rifle that people use. Though I used an M4, the results should be the same for most other rifles that have the same cylinder type (full cylinder) and same style of bucking.

So here is a list of parts in the rifle for your information:

E&C M4

E&C M4 Gearbox Case

E&C 18:1 Standard Gears

E&C Full Cylinder

E&C Piston Head

SHS Piston with Full Metal Teeth(Piston and Piston head weigh 27gms)

Super Shooter M160 Spring

E&C For all other little gearbox parts

E&C M4 Standard Metal Hopup Chamber

E&C M4 Standard Bucking (About 60 durometer)

SHS High Torque Motor

SHS M4 Air Nozzle (O-ring gives better air seal)

Action Army 550mm x 6.03mm Inner Barrel

M4 Mag Well Lock/Brace (Helps to absorb shock from front of gearbox)

11.1v 2200mah Lipo

**So here is what you have been waiting for. The barrel lengths that gave the most energy for each bb weight were:**

BB weight Barrel Length for a 6.03mm barrel

0.2gm---------------435mm

0.23gm--------------430mm

0.25gm--------------425mm

0.28gm--------------420mm

0.30gm--------------415mm

0.32gm--------------400mm

0.36gm--------------380mm

0.40gm--------------355mm

0.43gm--------------345mm

0.45gm--------------335mm

If you have a longer nub in your packing/bucking (so R-hop), then your nub will not be protruding as far into the barrel and the bb will be able to get past easier. So you are not wasting as much air from the cylinder. In this case, you can use a slightly longer barrel. AEG's are pretty forgiving though and so 50mm either side of these recommended barrel lengths will not really make too much of a difference. You should only lose about 0.05 of a joule.

NOTE: I have also noticed that when using a 6.01mm barrel, the barrel length can be increased by up to about 10% depending on the bb weight used. The lighter the bb, the longer you can go. This would be due to less air wastage.

**Here are the specs for a full cylinder:**

Diameter = 23.8mm,

Length of cylinder = 72.5mm,

Cylinder head area = 5mm,

Piston Head Area = 7mm

Compression area minus piston head and cylinder head = 60.5mm

Usable compression volume = 26915mm

BB weight-----Barrel Length-- Barrel Volume----- Ratio

0.2gm------------435mm---------12423mm3------2.17:1

0.23gm----------430mm---------12280mm3-------2.19:1

0.25gm----------425mm---------12137mm3-------2.22:1

0.28gm----------420mm---------11994mm3-------2.24:1

0.30gm----------415mm---------11851mm3-------2.27:1

0.32gm----------400mm---------11423mm3-------2.36:1

0.36gm----------380mm---------10852mm3-------2.48:1

0.40gm----------355mm---------10138mm3-------2.65:1

0.43gm----------345mm----------9852mm3-------2.73:1

0.45gm----------335mm----------9567mm3-------2.81:1

__USING PORTED CYLINDERS__The advantage to these ratios is that you can now use ported cylinders and match it with the correct barrel length just by doing 3 simple calculations and then you should be able to achieve the best efficiency for your ported cylinder.

**WORKING OUT CYLINDER VOLUMES ON PORTED CYLINDERS**

First work out your actual compression volume of the ported cylinder. This is the entire area from the front of your port to the cylinder head. Let's say I had a ported cylinder that was 32mm measured from the front of the cylinder, I would then minus 5mm for the cylinder head. So it would have a compression distance of 27mm. Next I need to know the radius of my cylinder. This is the diameter (23.8mm) divided by 2. Which is 11.9mm. Then we need to do a simple mathematical calculation to work out the area of the cylinder using pye x radius x radius x length.

So this is it:

3.141 x 11.9mm x 11.9mm x 27mm = 12009mm

So our cylinder volume is 12009mm

**WORKING OUT THE BARREL VOLUME**

Now we need to work out what the barrel volume would be by using the correct ratio from that bb weight that we will be using and the cylinder volume measurement. Let's say I was using a 0.25gm bb, the ratio I should be using is 2.22:1. So the calculation is simply. Cylinder Volume divided by 2.22 = Barrel Volume

So this is it:

12009mm divided by 2.22 = 5409mm

So our barrel volume is 5409mm

**WORKING OUT THE CORRECT BARREL LENGTH**

I have already decided to use 6.03mm barrels as I like tight bore barrels more. So I know that my radius is 3.015mm. Now by using the barrel volume we need to work out how long the barrel will be. This is done with a few more small calculations.

Barrel Volume ÷ (barrel radius x barrel radius) ÷ pye = barrel length.

So, 5409mm ÷ (3.015mm x 3.015mm)÷ 3.141 = 189mm. So the best barrel length using the ported cylinder and the 2.22:1 ratio with a 6.03mm barrel is 189mm.

If you are not keen on all the maths then just use this website. At the bottom there is a Cylinder to Barrel Ratio Calculator: AirsoftTech.dk - Calculator to calculate, Speed, Rate of Fire, Gearsets, etc.

NOTE: This calculator says that the cylinder diameter is 23.5mm. Most cylinders are actually 23.8mm. So make sure you change it.

Here are the most efficient ratios for a

**V2.5**gearbox (

**SR25**)

SR25 Cylinder Ratios using 6.03mm barrels

BB weight-----Barrel Length-- Barrel Volume----- Ratio

0.2gm------------505mm---------14336mm3------2.17:1

0.23gm----------502mm---------14193mm3-------2.19:1

0.25gm----------495mm---------13993mm3-------2.22:1

0.28gm----------490mm---------13908mm3-------2.24:1

0.30gm----------484mm---------13708mm3-------2.27:1

0.32gm----------467mm---------13279mm3-------2.36:1

0.36gm----------442mm---------12565mm3-------2.48:1

0.40gm----------414mm---------11709mm3-------2.65:1

0.43gm----------402mm---------11423mm3-------2.73:1

0.45gm----------391mm---------11080mm3-------2.81:1

Happy teching