Low fps after installing a shorter barrel

Discussion in 'Gun Building, Modifications & Repairs' started by linkster145, Aug 3, 2012.

  1. linkster145

    linkster145 New Member

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    Had a normal M4 AEG with a 509mm barrel installed and an unported cylinder, I decided to make it a CQB gun by installing a shorter barrel that was 150mm long, the FPS suddenly dropped from around 400 fps to 130 (with .20's). I thought this was due to to uneven cylinder to barrel volume ratio. I calculated the volumes and drilled holes into the cylinder making a 2:1 cylinder to barrel volume ratio. I also added a Systema bucking and H-nub and the fps rose to 160 fps. I have great compression in the gearbox (double o-ring cylinder head, o-ring air nozzle, ect.) and so this is really puzzling me. Any suggestions to raise the abnormally low FPS?
     
  2. Kmkraft12

    Kmkraft12 New Member

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    I know you say you have great compression because of the things in the gun, but have you checked it. Try the floss or teflon tape mod. I could be the BB weight also. A .30 will take a much longer amount of time to accelerate than a .20 will. With a shorter barrel you generally have to use lighter bbs.
     
    Last edited: Aug 3, 2012

  3. Lefse

    Lefse Well-Known Member Supporting Member

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    Barrels that short are simply a ***** when it comes to compression. I almost gave up on my MP5K with a 140mm barrel, gun with barrels that short are just extremely sensitive, everything have to be 100% right to make it work properly. You just have to go through everything several times, removing every potential air leak and bottleneck.
     
  4. SilentVectorX

    SilentVectorX Member

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    San Angelo
    130 FPS with .20g *is* abnormally low.

    I installed a 110mm Marui barrel into my ICS M4 (Project DEMONHAMMER), specifically for the desired effect of drastically dropping the muzzle velocity. With an M120, full cylinder, and lightweight piston assembly, I got 290 FPS.

    You may want to check the weight of your piston assembly, make sure the Air Nozzle is making a good seal with the hopup bucking lips, and make sure the bucking is sealing well with the barrel (seal with gasket maker if necessary).
     
  5. phyre

    phyre Member

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    Richmond
    +1. This can make all the difference in the world.
     
  6. linkster145

    linkster145 New Member

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    I have checked compression by pushing the piston forward in the cylinder with the air nozzle on, and the piston won't go all the way forward. I have also done the teflon tape mod and blown through the barrel and no air escaped through the the bucking/hop-up , so i believe I have good compression.also I always use .2's. Nortech your'e right, a bunch of forums have people complaining about this same problem in mp5k's. I would like to know if the nozzle is making a good seal with the bucking lips, but how can I test whether it is or not? also I have an aluminum piston head because I thought that would boost fps a bit because it gives added momentum to the forward moving piston, but would it help to switch to a plastic piston head?
     
  7. alex

    alex New Member

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    Atlanta
    quick compression testing:
    tappet plate and spring needs to be installed

    [ame="http://www.youtube.com/watch?v=MFnUjxdiXuU"]http://www.youtube.com/watch?v=MFnUjxdiXuU[/ame]
     
  8. linkster145

    linkster145 New Member

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    thanks for the help guys, i fixed the compression with a variety of mods and now I get 330 fps with a low quality m120. (not great, but very satisfying compared to 130 fps and perfect for cqb)