M16 fires without pulling the trigger

Discussion in 'Gun Building, Modifications & Repairs' started by ncore, Dec 7, 2012.

  1. ncore

    ncore New Member

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    Ok, so I have been having a bunch of problems with my gun, the latest being the gun firing without pulling the trigger.

    I have it hooked up to a homemade mosfet. I opened up the gearbox and checked the trigger contacts. No archs whatsoever. I even took out the little piece of the trigger contact that slides back and forth, and also the side plate that holds the one contact. I hooked it up to a motor outside the gearbox, connected the battery, and the motor turned on.

    So after inspection, I concluded that it had nothing to do with anything inside the gearbox.

    I then proceeded to take apart the mosfet, and build a brand new one from scratch. After putting it all together, the same thing happened, and the gun fired when on safe.

    This is really bugging me. When I first installed the first mosfet, everything worked fine and now all of a sudden its not working.

    I appreciate all help, thanks!
     
  2. S0m3_0n3

    S0m3_0n3 New Member

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    you could just have wired both of your mosfets wrong, they could be bypassing your trigger switch and going straight to your motor
     
    Last edited: Dec 7, 2012

  3. JakeBworth

    JakeBworth New Member

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    Hoboville
    THis. ^^^^^

    BTW, I find this problem incredibly funny, yet dangerous at the same time.
     
  4. ncore

    ncore New Member

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    But like I said, the first mosfet worked fine for a couple days.
     
  5. S0m3_0n3

    S0m3_0n3 New Member

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    missed that part, maybe it's the way you assemble your mosfets? is it possible that you're bending some contacts when you put it in the gun?
     
  6. ncore

    ncore New Member

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    Wait, what do you mean by bending contacts?
     
  7. bal1

    bal1 New Member

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    I had the same problem with the old saw I had,I think the tech said it was my wiring/trigger contacts. Others said my gun was possessed,seems yours is to.
     
  8. Ehudakineyah

    Ehudakineyah New Member

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    Is this a homemade MOSFET?

    image-1604071045.jpg

    Like these little things? Lol :)

    Make sure they're wired correctly :) and heat shrinked in the right places

    Did you say it only lasted a couple days? O.O
     
  9. Mystery43_4

    Mystery43_4 New Member

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    It sounds like you blew your first one, and when you were making your second one, either you over heated it, or didn't protect it properly against ESD. Make up another one making sure to pay attention to how long you hold the soldering iron to the FET. Also check your wiring for anywhere that it can short.
     
  10. Ehudakineyah

    Ehudakineyah New Member

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    Make sure the FET is not building up heat at all lol. It barely. Should be "touchable" in lamest terms from my instructor lol xD

    Or you can use a micro copper alligator clip to disperse heat if you don't have a high enough watt iron.
    You should be using about a 30 watt and higher iron. I would stay under 160 watt cause I blew some 1/4 watt resistors just by touching it lol.
     
  11. Mystery43_4

    Mystery43_4 New Member

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    30-50w works great.

    I have an adjustable 40w Weller soldering station, and I find that for most cases I have the knob cranked to about 3.5-4 (1-6, low-high). 5 and up to heat up the iron quick, and to do large gauge wire, 12+.

    Alligator clips are good, they actually sell heat conductors for this exact purpose. They're the same as the alligator clips, minus the teeth.
     
  12. ncore

    ncore New Member

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    Kind of, but I made it myself.

    Ok I will make another one and insure I don't use the soldering iron on it for too long.
     
  13. jimfc3

    jimfc3 New Member

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    evergreen park
    Did you twist your battery leads and the trigger leads??
     
  14. dylan484

    dylan484 New Member

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    Tifton
    A picture of your MOSFET would probably help us as well.
     
  15. ncore

    ncore New Member

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    What do you mean by this?

    I will try, but I will be testing for a few hours and might not be able to post them right away.
     
  16. Mystery43_4

    Mystery43_4 New Member

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  17. ncore

    ncore New Member

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    So I built a completely new MOSFET and the same thing happened again...

    Idk if I missed when I opened up the gearbox to check the wiring and trigger contacts.

    I guess I will try to rewire everything...

    Would you guys suggest going ahead and buying a new set of trigger contacts?
     
  18. ncore

    ncore New Member

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    Also, I forgot to mention that the trigger selector spring came out.

    But when the trigger box is in the lower receiver, putting the gun on safe stops the trigger like it should, but like mentioned, the gun still shoots full auto anyway.

    I am pretty sure the function of this spring is just to insure that the gearbox is setto full auto when not in the receiver, but that should not be a cause for the gun to fire full auto when no trigger is pulled.

    I ended up buying a new set of trigger contacts and when it gets here, I will completely rewire the gun again.
     
  19. Ehudakineyah

    Ehudakineyah New Member

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    Alrights :) keep us updated man
     
  20. Edonkulation

    Edonkulation New Member

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    Not sure if it helps, but I had an issue one time where because of the way I ran the wires into the buttstock of my AR and when I put the battery in, I had a runaway gun. It turns out that, I had two wires bunched up and when I put the back plate and buffer tube on, it was squeezing the wires and caused my gun to go crazy.