Yesterday, a friend of mine had a bit of a problem with his new (used) AR15 lower - it was doubling the rate of intended fire!

Since this was the first time he shot it, it was a bit of a surprise.  

I BELIEVE the problem was in the trigger.  This was the general consensus of those there who looked at the lower and upper.  We believe that the bolt was riding over the hammer or the disconector wasn't working, even though it worked when the lower was observed by itself.

There are NO apparent mods the the trigger/sear or anything like that and it is definately NOT a burst kit or anything like that.  In fact, it was just sold after being in police evidence for 6 years.  The police had checked it all out for legality, then gave it back to the original owner who was too shaken up by the JBT squad to keep it.

He's already taken the trigger group out of the lower so there is no appearance of owning a FA gun.  However he wants your advice on whether it could be another part that is causing this situation that he should be aware of.

Other members of the board witnessed this and said he should post it here, but of course he was too afraid of the JBT squad showing up on his doorstep!

I just looked below and saw a similar problem in this thread:
[url]http://www.ar15.com/forums/topic.html?b=3&f=66&t=145416[/url]

Do you think he should just follow the same suggestions as in that thread?

Have him check the disconnector and the spring.   There is a chance that someone installed the disconnector spring upside down.  The larger end into the the trigger slot, not the smaller end.

If the spring install is correct, then have him order a new FCG blister pack, and be done with it.  Chances are that someone did a home brewed trigger job on the FCG, and didn't  have a clue.

If he wants to try and reset the disconnector/FCG to get it back in timing, lets us know and we can post the instruction, but if he has no smithing talent, then just replace the whole group.

Dano

Also check the firing pin tip and make sure the firing pin is not bent.-have seen a few double up with an improper tip

Considering it was in a police storage locker for 3 years, I decided to take it from him, take out and thoroughly clean the trigger parts, then reinstall them after carefully inspecting for any modifications that were done.

Upon inspection, I found that the hammer appeared to be shaven on the top, which looks like it occured naturally.  I think this is also why other uppers were not fitting the gun without the bolt to get stuck on the hammer.  

I then cleaned the shit out of everything, lubed it up and reinstalled it.  Then I dremelled the top of the hammer where the bolt was hanging up and that's working fine now also.  Now the disconnector is working normally.  He's going to test it out this sunday, so we'll see if it worked.

Next step is to replace everything, thanks for your help, it was scary shooting seeing this happen!

Forgive me for stating the obvious, but tell him to make sure he starts with no more than 2 rounds in the magazine when he tests it.  Only after he's done so several times without it going FA should he start loading more into the mags.

Way ahead of you!  No need to throw a 30 round mag downrange and find out the displeasures of anal intrusions![;)]

Balzac72,

Before he takes the rifle out again, you need to perform a function test on the rifle.
Chances are, the rifle is going to fail the tests.  Since you stated nothing about tuning the disconnector, the rifle will still have a problem.

HOW TO TEST.
Swing open the lower, and hold the trigger back.
Then cock the hammer to be retained by the disconnector.
Push the hammer all the way down, then let your thumb slide away(to the side) and see it the hammer is retained by the disconnector.
The hammer needs to hit the disconnector full force to check the engagement of the two.

The ever so slowly, release the trigger until the hammer is released from the disconnector, and retained by the trigger sear.
Here is where you check the real timing of the disconnector.  If the disconnector releases too soon in the return stroke, the hammer will be released, and miss the trigger sear. Hence releasing the hammer to fire another round on the return stroke.  
Since the normal reaction when firing a rifle is to squeeze the trigger. Then during the recoil to allow the your trigger finger to slightly dance on the trigger, with a badly timed disconnector, this will cause the auto firing.



Since you stated that the hammer never bound up, then either the hammer was bounced off the disconnector at carrier impact with the barrel/receiver, or the hammer was released on the forward stroke of the trigger and missed the sear.

Also, since I see that you wish to tune the rifle, instead of just changing out the FCG, here are the instruction.

The function of the disconnector is to retain the hammer until trigger resets.  By retaining the hammer, you want the release to be just short of trigger reset(trigger coming to rest al the way forward).  
The disconnector can be tuned by removing metal from it where it contacts the trigger.  By removing metal from the front foot where it contacts the front of the trigger, you are caming the disconnector forward, which in turn, makes the disconnector rotate farther forward.  Hence, this requires the disconnector/trigger to be  father forward before the hammer is released the disconnector.  The correct timing will allow the trigger sear to cam farther forward, and be in the correct location to to retain the hammer sear at release from the disconnector, instead of missing/deflecting off the hammer sear.

When removing metal from the disconnector foot, remember that the disconnector is attached to the hammer at a pivoting point. Which means that when you remove metal from the front foot, you will need to remove it in relationship to this the pivoting point.  If you just remove metal at the same angle of the current foot, them what will happen is that the new contact angle will be wrong, and only the tip of the foot will rest on the trigger's top edge.  
This leads to the new contact edge wearing down, and due to the added caming of the disconnector forward, the disconnector does not release the hammer, even when the trigger is at full rest.

Also, if the hammer sear is worn/tweaked, then this will also have to be considered in to the adjustment. If it is too worn/tweaked, then all the adjustment of the disconnector is not going to fix the FCG, due to the sear not having enough of a edge to allow the trigger sear to retain the hammer.

Or,
as Royce would say, just replace the FCG. For the $49.95 that it will cost you, it's a lot cheaper that having to deal with the BATF or replacing the receiver if you have a Kaboom.

Dano

Wow, now thats an explanation!  

I'm pretty good with trigger geometry and tuning, so I did check for most of what you mentioned when I was playing with it today.  The disconector was very strange and I actually think one of the springs was off before I took it apart.  Upon reassembly, everything acted normal like my AR lowers.  

I also think that you're right about the carrier's ride over disconnecting the hammer from the...well...disconnector.  After mildly shaving the hammer, it appears to be working perfectly.  Yes, it will be replaced, but my mechanical side wants me to test my work out to see if my alterations worked.  If not, the FCG will be yanked at the range.

Thanks for your help, my next question will probably be over how to install the Jewell in my Colt Elite when i get it.  Maybe i'll give him the Elite trigger since it's pretty light and smooth.

[url]http://www.highpowerrifle.com/Jewell.htm[/url]