Needing help with 9mm AR15.

FFLshooterFFLshooter MemberPosts: 461 Member
I’ll try to cut to the chase. I have a customers AR15 in 9mm conversion that he knows squat about and brought it to me saying it was running auto (but not runaway). At the moment I can’t go test fire it at the gun club I belong too, but these issues are usually straight forward, except for this time. I check the obvious first. All parts are brand new, springs in correctly, disconnector working properly, sear looks good, etc. I would pull the charging handle, pull trigger, let the handle fly, yada yada. Nothing. This is a new build but I can’t find a name or number on anything except the Anderson lower. I even thought maybe it was slam firing but the firing pin is free and clean and the hammer can’t strip a round out of the mag and ride the bolt home either because the sear catches well before that. The bolt is ramped and the hammer is your standard AR hammer. I even weighed the buffer at 8ounces and the bolt at 12.5 and that seems about right. I just can’t find a reason that this thing would run auto AT ALL. Any recommendations or suggestions will be greatly appreciated. Also don’t have much experience with an AR in a blowback configuration.

Replies

  • FisheadgibFisheadgib Senior Member Posts: 5,209 Senior Member
    Eli had one at the SE shoot that ran like a top and he might be qualified to help you. (That thing was a hoot to shoot and put a smile on a lot of faces.)
    snake284 wrote: »
    For my point of view, cpj is a lot like me
    .
  • BAMAAKBAMAAK Senior Member Posts: 4,199 Senior Member
    Try swapping the trigger?  Know a guy that had a brand 1500.00 JP that would do 2 or 3 round bursts.  Never heard what it was though.
    "He only earns his freedom and his life Who takes them every day by storm."

    -- Johann Wolfgang von Goethe, German writer and politician
  • tennmiketennmike Senior Member Posts: 22,973 Senior Member
    The only two things that come to mind is a sear spring that is weak/broken, and insufficient sear/hammer engagement. Both will cause the hammer/sear surfaces to disengage as the bolt slams into battery. That's generic to any AR configuration.
    One test is to remove the upper and on a WELL PADDED  bench, bang the front of receiver into the pad with the hammer cocked. Do it several times, and drop and reset the hammer every time. If it's the sear/hammer engagement, the hammer will drop. And that could be caused either by a weak/broken sear spring, or insufficient hammer/sear engagement.
    If a Liberal throws a hand grenade at you, pick it up, pull the pin, and throw it back at them.



  • MichakavMichakav Senior Member Posts: 2,201 Senior Member
    edited June 9 #5
    Disconnector/spring. Had a new build go full on me recently because one of the coils collapsed. Make sure the spring is good, not hanging up, oriented properly (fat side down) and the disconnector is in spec.. It was quite the surprise to me and the 2 buddies I was shooting with. Should have seen there eyes...lmao...New spring for mine and all is right in the world.
  • FFLshooterFFLshooter Member Posts: 461 Member
    After some more dicking around and pulling the hammer back to full travel (with upper removed, obviously) and letting it snap forward with my finger nowhere near the trigger, I finally got the thing to trip. A closer look reveals what feels like a weak disconnector spring, but I’m going to suggest to the customer that I replace the whole FCG. It looks like cheap junk anyway. Thanks all for the suggestions.
  • MichakavMichakav Senior Member Posts: 2,201 Senior Member
    After some more dicking around and pulling the hammer back to full travel (with upper removed, obviously) and letting it snap forward with my finger nowhere near the trigger, I finally got the thing to trip. A closer look reveals what feels like a weak disconnector spring, but I’m going to suggest to the customer that I replace the whole FCG. It looks like cheap junk anyway. Thanks all for the suggestions.

    I was pretty positive that would be the culprit.
  • tennmiketennmike Senior Member Posts: 22,973 Senior Member
    There is a small coil spring in the trigger group of the SKS carbine that will cause them to go full auto when it either breaks, or stays compressed from not having the trigger group cleaned of all cosmoline/grease before firing. It's in the front of the trigger group and on the bottom right. Little spring sure can cause some excitement.
    If a Liberal throws a hand grenade at you, pick it up, pull the pin, and throw it back at them.



  • MichakavMichakav Senior Member Posts: 2,201 Senior Member
    edited June 11 #9
    tennmike said:
    There is a small coil spring in the trigger group of the SKS carbine that will cause them to go full auto when it either breaks, or stays compressed from not having the trigger group cleaned of all cosmoline/grease before firing. It's in the front of the trigger group and on the bottom right. Little spring sure can cause some excitement.

    It was definitely exciting in a way. Like you did something bad and got away with it....lol.. Luckily there is a 5 round limit at that range (a local club) and it was just us. Rifle immediately got put away and no sirens approached. We were waiting for them because there was a bunch of people fishing around the lake. 
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