What Barrel Length goes with what bb weight in an AEG?

Discussion in 'Gun Building, Modifications & Repairs' started by 1tonne, Aug 12, 2018.

  1. 1tonne

    1tonne Well-Known Member

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    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
    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 = 26241mm
    [​IMG]

    Cylinder Ratios
    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
     
    Last edited: Nov 30, 2019
    Chow, Belgrim, Airsup and 15 others like this.
  2. 1tonne

    1tonne Well-Known Member

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  3. link0

    link0 Well-Known Member

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    Great info! I'd like to point out that most of us will AOE correct the cylinder head, which will reduce the cylinder volume fairly significantly. This only affects your optimal barrel length chart.
     
    1tonne likes this.
  4. 1tonne

    1tonne Well-Known Member

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    Yes. Peoples cylinder volumes may be different because of AOE or a thick layer of Sorbathane on the cylinder head but they can easily work out the correct barrel length by using the ratio for their given bb weight.
     
  5. Lefse

    Lefse Well-Known Member Supporting Member

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    This is very useful information, good job. :)
     
  6. cetane

    cetane Well-Known Member

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    Nice. So my regular tactic of using a full cylinder with basically everything is sound lol (well, mostly with all the generic 363mm barrels I have)
     
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  7. antizombie

    antizombie Well-Known Member

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    That was a great read. I always really love seeing original content that someone obviously worked very hard on. If this was reddit I would give you gold.
     
  8. 1tonne

    1tonne Well-Known Member

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    It is also OK to use a shorter barrel too. So if you like the look of short guns that is OK. The gearbox will just be working harder to get the same energy. So you do not have to stick with the recommended barrel lengths. They are the most efficient lengths though.
     
    cetane and link0 like this.
  9. Doc_Larsen

    Doc_Larsen Active Member

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    I love this post! I scour the internet for any and all solid Airsoft research supported by data. I hope one day to be in a position where I can add to the current body of knowledge. I also am on airsoftsniperforum and am a big fan of your research into the BASR platform. Thanks again for the impressive attention to detail and willingness to share hard earned information!:D
     
    1tonne likes this.
  10. Overkill777

    Overkill777 New Member

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    You said you tested 550mm barrel down to a few mm. Where is the rest of the data?

    Also I would like to see the raw data. I greatly appreciate the work you put into this and thank you for sharing.
     
  11. 1tonne

    1tonne Well-Known Member

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    I am not handing out the raw data at the moment as I would like to keep some secrets to myself. I will in time (probably in about a year) release it.
    The raw data shows the best barrels lengths for efficiency but also the trend of joule creep. (But this is not getting release yet)
     
  12. link0

    link0 Well-Known Member

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    Have you also tested whether the barrel length giving the highest efficiency also gives the highest accuracy? If not, is slightly shorter or longer than the most efficient length more accurate?
     
  13. 1tonne

    1tonne Well-Known Member

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    From what I have seen, if the energy levels and bb weight are the same with an over volumed (short barrel) and a match volumed (perfect barrel length) rifle, then the accuracy will be about the same. Once you start to under volume (Barrel too long) a rifle though, that is when you start to lose accuracy. AEG,s are pretty forgiving though and most of the time people do not use extra heavy weight bb's in them which can contribute to under voluming.
    As an experiment I massively under volumed a rifle once. I had a VSR10 with a 600mm inner barrel and I fired 0.45gm though it. They would only fly about 20 meters and then they flung off in any direction.

    NOTE: In my DMR, I use an inner barrel of only 170mm (Not an efficiency build). I also used a short 1 inch outer barrel and made a 500mm silencer for it. Muzzle report is very low.
     
  14. link0

    link0 Well-Known Member

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    Great info. Thank you!

    I'll try to err to the side of overvoluming.
     
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  15. Honeybadgerz17

    Honeybadgerz17 Well-Known Member

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    Very cool info! I think @BOA_SP3CT3R was going to conduct an expirememnt similar to this! Maybe you could compare data if he goes through with it
     
  16. Splode

    Splode Well-Known Member

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    So a lighter weight bb has higher energy than a heavier bb in a long barrel? Im confused. I thought heavy bbs were better for long barrels.. :S
     
  17. link0

    link0 Well-Known Member

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    Heavier BBS take longer to pass through a barrel because they are moving slower and therefore take more air volume from the cylinder.
     
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  18. cetane

    cetane Well-Known Member

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    You have a set amount of air in your cylinder.

    Heavier bb needs more of that air to get moving.

    More air is used for heavier bb, so barrel needs to be shorter to compensate.

    Most commonly used weights seem to be from 0.25g-0.32g. Above that weight bbs start getting expensive. Below that good for cqb/indoor.
     
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  19. Splode

    Splode Well-Known Member

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    But assuming you do have that cylinder volume, heavier bbs have more energy in a longer barrel?
     
  20. 1tonne

    1tonne Well-Known Member

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    Once the barrel length goes over the lengths that I have recommended, then you will start to lose a little energy. Though if you go over by about 50mm you will only be losing about 0.05 of a joule which is nothing to really worry about.
    Also, most people do not use heavy weights in their AEG's. With the lighter bb's, the loss of energy is not much but with heavy bb's the loss is more noticeable. But that is with 0.36gm bb's or above and so with these weights you should aim more for the correct barrel length.