The Kitchen Table Gunsmith
  
   
Main menu  
 

 
 

 

The Case of the Double-Tapping AR10
by Roy Seifert


Click here to purchase a CD with this and all Kitchen Table Gunsmith Articles.

 

Disclaimer:  This article is for entertainment only and is not to be used in lieu of a qualified gunsmith.  Please defer all firearms work to a qualified gunsmith.  Any loads mentioned in this article are my loads for my guns and have been carefully worked up using established guidelines and special tools.  The author assumes no responsibility or liability for use of these loads, or use or misuse of this article.  Please note that I am not a professional gunsmith, just a shooting enthusiast and hobbyist, as well as a tinkerer.  This article explains work that I performed to my guns without the assistance of a qualified gunsmith.  Some procedures described in this article require special tools and cannot/should not be performed without them.

Warning:  Disassembling and tinkering with your firearm may void the warranty.  I claim no responsibility for use or misuse of this article.  Again, this article is for entertainment purposes only!

Tools and firearms are the trademark/service mark or registered trademark of their respective manufacturers.

Introduction
One of my long-time shooting buddies, Dave, brought me an AR10 that was built by a different friend, James, from commercially available parts.  James had purchased a bare, serialized lower and had it transferred through his FFL dealer.  He then purchased a lower parts kit and assembled his lower.  When Dave attempted to shoot this home-built AR10, sometimes when he pulled the trigger the rifle would fire two rounds, not just one!  After about the third time this happened Dave stopped shooting the rifle and brought the lower receiver to me to have a look.

Initial Inspection
I first wanted to test the function of the disconnector.  With my finger pressing the trigger I pulled the hammer back so it would engage the disconnector.  With my thumb on the hammer to prevent it from hitting the frame, I released the trigger.  The disconnector released the hammer and it should have been caught by the sear, but the hammer completely fell to the front of the receiver, which would have caused a second round to be fired.  With my finger off the trigger, when I pulled the hammer back the sear caught the trigger. 

Based on this initial inspection, hereís what was happening:

  1. Dave pulled the trigger.

  2. The sear released the hammer which fired a shot. 

  3. The bolt being driven back cocked the hammer which was caught by the disconnector. 

  4. Dave released the trigger which caused the disconnector to release the hammer. 

  5. The hammer missed the sear which caused the rifle to fire a second time causing the double-tap. 

  6. When the bolt came back the second time the hammer was caught by the sear because Dave had let off the trigger.

 

I discovered that the trigger pin was not seated fully in the receiver.  It had slipped out of the left hole and was protruding from the right side of the receiver causing the trigger and disconnector to be crooked.  The trigger and disconnector being crooked in the receiver prevented the disconnector and/or sear from catching the hammer.  Ok, so what caused the trigger pin to come out of the receiver?

 

The hammer and trigger pins are exactly the same for an AR10 (and AR15 by the way).  The pin has two grooves to prevent it from moving once seated in the receiver.  When the pin is used as the hammer pivot pin, there is a small spring in the center of the hammer that fits in the middle groove of the pin to prevent it from moving.  When the pin is used as the trigger/disconnector pivot pin, one leg of the hammer spring sits in the outside groove to prevent the pin from moving.  Notice how the grooves are rounded; a tap on the pin with a punch will unseat the pin.  On Jamesí rifle I noticed that the hammer spring leg was not setting in the groove of the trigger pin.

I removed the hammer, trigger and disconnector from the receiver and compared them to a set of AR15 trigger parts I had left over from when I replaced the trigger group on my own AR15.  I immediately noticed that the hammer spring had been installed backwards on Jamesí rifle!  This caused the leg to be set at such an angle that it would not contact the groove in the pin.  This was also causing the spring to be flexed backwards. 

I installed the spring correctly, reinstalled the trigger parts and found that the trigger/disconnector pin now would not move because the leg of the hammer spring was sitting in the groove correctly.  When manually cycling the hammer with the trigger pulled, the hammer engaged the disconnector.  When I released the trigger the hammer was caught by the sear indicating the parts were now working correctly.

Range Session
I went with Dave to the range to test the rifle.  I loaded two rounds in a magazine to make sure the trigger group was functioning correctly.  After each shot, I held the trigger back, then carefully released it to ensure the disconnector and sear were working properly.  I could both hear and feel the disconnector release the hammer and be caught by the sear.  I then loaded 5 rounds in a magazine and the rifle continued to function properly.  Problem fixed!

Summary
Iím all for building guns from parts and often times to fix a gun all that is needed is a replacement part.  However, when working on any gun I always have a schematic (exploded parts view) available to help with proper orientation and location of parts.  If a schematic is not available I use a digital camera and take photos as I disassemble the gun so I know how to reassemble it.  As for installing a part backwards; Iíve done it many times and will probably do it again!  We learn through our mistakes.  To James credit he built his rifle from parts having never disassembled one, and it really is a beautiful, and now correctly functioning, rifle.

 

 
   © Copyright 2015 Roy Seifert.