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Technosavant · June 25, 2013, 10:02 AM

Did it pass the function checks?

If his response is "What function checks?" then he missed a major part in making sure the thing is safe to even take to the range.

AR fire control group function check procedure*:
1) Cock the hammer, turn the safety on. Pull the trigger. The hammer should NOT fall.
2) Turn the safety off. Pull the trigger. The hammer SHOULD fall.
3) With the trigger still depressed, cock the hammer. The hammer should be held by the disconnector.
4) Release the trigger. The disconnector should release the hammer to be caught by the trigger.

*I prefer to do this with the upper off the lower so I can see what is going on, but DO NOT let the hammer just fly forward and hit the receiver- that can damage things.

I had a fire control group from a reputable manufacturer (Daniel Defense) fail at #4- the trigger would not catch the hammer. That would mean you'd have it firing twice with each trigger pull/release. Not a safe condition. Turned out that it was two parts on the ends of the tolerance specs... any other hammer or trigger and it worked fine. That particular hammer/trigger together... problem. Daniel Defense is a stand up company and replaced the hammer, trigger, and disconnector for me for free. The replacement unit did work fine.

Now, it's not a guarantee that this is your problem, but if the fire control group passes these checks then the issue isn't in your lower.

You can then check out the bolt carrier group. I wonder if there isn't some issue in the firing pin (or the firing pin channel) keeping the pin from floating to the rear when the action locks closed (there's no spring keeping the pin off the primer, dented primers in an AR from just cycling the thing without firing are not unusual.

If you had a situation like I had, then the slam fire would have been from letting off the trigger and the dented primer on the 3rd round would be normal. If you have an issue with the firing pin/bolt carrier then for whatever reason the firing pin floated free and kept it from burning through the whole mag on full auto.

June 25, 2013, 10:02 AM	#4
Technosavant Senior Member Join Date: May 29, 2007 Location: St. Louis, MO area Posts: 4,040	Did it pass the function checks? If his response is "What function checks?" then he missed a major part in making sure the thing is safe to even take to the range. AR fire control group function check procedure: 1) Cock the hammer, turn the safety on. Pull the trigger. The hammer should NOT fall. 2) Turn the safety off. Pull the trigger. The hammer SHOULD fall. 3) With the trigger still depressed, cock the hammer. The hammer should be held by the disconnector. 4) Release the trigger. The disconnector should release the hammer to be caught by the trigger. I prefer to do this with the upper off the lower so I can see what is going on, but DO NOT let the hammer just fly forward and hit the receiver- that can damage things. I had a fire control group from a reputable manufacturer (Daniel Defense) fail at #4- the trigger would not catch the hammer. That would mean you'd have it firing twice with each trigger pull/release. Not a safe condition. Turned out that it was two parts on the ends of the tolerance specs... any other hammer or trigger and it worked fine. That particular hammer/trigger together... problem. Daniel Defense is a stand up company and replaced the hammer, trigger, and disconnector for me for free. The replacement unit did work fine. Now, it's not a guarantee that this is your problem, but if the fire control group passes these checks then the issue isn't in your lower. You can then check out the bolt carrier group. I wonder if there isn't some issue in the firing pin (or the firing pin channel) keeping the pin from floating to the rear when the action locks closed (there's no spring keeping the pin off the primer, dented primers in an AR from just cycling the thing without firing are not unusual. If you had a situation like I had, then the slam fire would have been from letting off the trigger and the dented primer on the 3rd round would be normal. If you have an issue with the firing pin/bolt carrier then for whatever reason the firing pin floated free and kept it from burning through the whole mag on full auto.