Low fps after installing a shorter barrel

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

  1. linkster145

    linkster145 New Member

    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

    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

    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

    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

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

    linkster145 New Member

    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

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

  8. linkster145

    linkster145 New Member

    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)