The last few times I used my AR it would occasionally “double”. I thought maybe it was just dirty and cleaned it. It didn’t do it for a while then it did it again. I cleaned it then when I hadthe private party last month it did it when it only had a few rounds after being cleaned. I set the gun aside and used a different one.
About three weeks ago I removed the trigger group and was going to replace the springs. That surely was the problem, right?
I was dismayed at the state of the important surfaces.If you are not a shooter, you may not recognize the term "double." This means that you pull the trigger, and you get two shots fired, not one. This might sound really cool--sort of a burst action rifle--but it is a serious problem, both because you are likely not expecting it, and because the BATFE has, on some occasions, prosecuted people with a rifle that would occasionally double for possessing a full auto weapon. This is a very serious crime. You do not want this to happen.
In this case, the problem was a worn surface on the hammer and trigger, probably caused by bad heat treating of the steel.
One of the comments on The View From North Central Idaho describes a test that people who spend a lot of time shooting AR-15s should perform every 90 days:
every few hundred rounds perform this function check.
Place selector on SEMI
With Trigger pulled to rear, pull Charging Handle and release.
SLOWLY (as in 1/4 the speed of normal) release Trigger.
Hammer should not fall, should release from disconnector and should ‘catch’ on trigger.
You should hear a ‘click’ but this is the sound of the Hammer being caught by the trigger.
Pull Charging Handle to rear and keep it pulled to rear
Pull Trigger to rear and while keeping trigger pulled to rear, release Charging Handle.
Repeat the SLOW Trigger release.
Repeat this three more times.
The Hammer must hold without falling for five times in a row while the Trigger is being slowly released.This is essentially the M16 function test the government performs.