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Discussion Starter · #1 · (Edited)
This is airsoft gold. A gift from 1Tonne to all the techs out there.

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
 

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Discussion Starter · #11 ·
I wonder if a revisit to the PDI vacuum piston head for AEGs in a DSG build would be of worth?
I don't think it would be worth it (could be wrong). An AEG piston o-ring releases it's pressure once it has stopped moving forward and air can come from behind it. That is pretty much the same as the PDI Vacuum Piston.
 

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Discussion Starter · #18 · (Edited)
They would have different performance.
Using a ported cylinder with the correct barrel length will mean you will lose energy compared to a full cylinder with a long barrel. The reason being is that a full cylinder has the entire barrel length to accelerate. In a ported cylinder the bb only starts to move once the piston has past the port. So the BB does not have the same acceleration time in the barrel as a full cylinder to barrel ratio would.
To get it back up to the original energy of a full cylinder to barrel ratio, you would then need to put in a bigger spring. This will put more stress on the gearbox and lessen trigger response.
 

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Discussion Starter · #20 ·
This Data is for an AEG. 550mm is too long. If you do not understand, get around the 400mm mark as this is a pretty good all round length.
 

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Discussion Starter · #22 ·
Yip. All correct.
Short barrels do not effect accuracy by the way. Though they may produce less energy and can be over volumed.
 

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Discussion Starter · #24 ·
It depends on the bb weight. A heavier bb will stabilize quicker and so if you use a shorter barrel you need to change to a heavier bb.
Also, if the cylinder is ported correctly, you can get very close to the maximum acceleration. A full cylinder with the correct barrel ratio will always get more energy though but ported will be close.
Another advantage with shorter barrels is that they make it easier to put backspin on heavier bb's while longer barrels make it harder to put backspin onto a heavy bb. Short barrels have the advantage that if the cylinder is ported, the piston has time to accelerate and then when the piston goes into the compression zone, it is already at a very good speed and this creates a pressure spike that gets the heavy bb passed the nub. With Longer barrels that have a full cylinder, there is not much of the spike when the piston starts to move. So I have seen people say that when they use heavy bb's the bb will get stuck behind the nub. Shorter barrel have less likely hood of doing this.
 

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Discussion Starter · #26 ·
The bb getting stuck behind the nub happens more when you have a low fps build but want to shoot a heavy bb. So not so much with high fps DMR builds. It happens more in long assault rifles (around 350fps).
 

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Discussion Starter · #28 ·
No. Not always but I will go into that another time. There are good reasons.
 

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Discussion Starter · #29 ·
No. Not always but I will go into that another time. There are good reasons.
OK. So I can go into this just a little. (Our club now chronos using joules. They just changed in the last couple of weeks and I did not want to say too much about it since I wanted to keep my advantage. I don't have the advantage now and so I can say more)
The simple answer is, that it depends if you are allowed to joule creep.
An AEG chronoing on 0.2gm bb's and firing at 350fps (1.13j) can be made for fire ridiculously high energy rates. (I never JC'ed with a Full Auto as I think that 1.13 joules is plenty for CQB)
Here is a build I made (did not use in game) that fired 350fps (1.13 joules) but registered at 2.1 joules with 0.43gm bb's

117mm x 6.00mm inner barrel (Tighter barrels are better for JC)
Full Cylinder
Piston weight (I think it was about 60gm)
Spring: I think it was an M165 or M170
BB weight 0.43gm
 

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Discussion Starter · #30 ·
Oh. By the way, I have even created a 300% gain in energy while joule creeping a rifle but the inner barrel was that short that even with the heavy bb, the energy was not enough to make it worth using in game.


Here is another experiment that people may or may not want to do.
M165 Spring.
Normal Plastic Piston with no weight
40mm x 6.01 inner barrel
Produces 187fps on 0.2gm bb's (0.3joules. So not much)
Produced 208fps on 0.45gm bb's (0.9joules. So 3 times as much)


You could put in a bigger spring to make it more gameable but an M165 is already pretty big. You could also increase the difference in energy rates by adding weight to the piston
 

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Discussion Starter · #32 ·
Yip. Shorten the barrel and move the port forward stuff like that.
Or, put in a bigger spring and shorten the barrel.
One variable goes up and the other goes down.


You should check the energy rating of the 0.36gm bb with the 18" (450mm) barrel and then put in a 380mm barrel. It should make the smallest gain. Probably only about 0.05 joules though. So it is meh.
 

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Discussion Starter · #34 ·
1.5 joules, I would use a 0.36gm bb and a 6.01mm barrel that is about 480mm will be somewhere around the most efficient ratio with a full cylinder. You could use a shorter barrel and also port the cylinder if need be.
There are minor variables that can change the ratios mentioned but overall if you have a 100% air seal, the barrel lengths should be about right. I guess the biggest variable is inner barrel diameter.
 

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Discussion Starter · #36 ·
I match all of my builds. My main M4 has a barrel length of about 190mm and with the port in the correct position, it works great. At 1.13j I use 0.28gm
 

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Discussion Starter · #39 ·
For 0.32gm in a 6.01mm barrel = Anywhere between 400-440mm. It can be a little bit of a guessing game and so I would most likely go for 440mm.
 

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Discussion Starter · #42 ·
Yip. Work out the barrel length using the ratios provided. So it will be a short barrel.
 

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Discussion Starter · #49 ·
There is only one pic and from what I can see, it is working. The pic just shows the area between the front of the piston and the cylinder head. So the compression area.
 
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