Had a new rifle double on me a few times today. Came home and spent over an hour finding the problem. Did the function check everything was fine. When I would charge the action the hammer kept following the carrier back. It would not stay locked on the disconnector even if I held the trigger back when I charged it and return it. Keep function checking it with the upper off and everything work perfectly.

If you think you have seen it all give me your guess as to what the problem was and I will post what it was later.

Lock the bolt back, hold the trigger (pulled), and hit the bolt release button.  Then slowly release the trigger.


If the hammer slips from the disconnector and strikes the firing pin (either tests), you will need to re-time the disconnector.

When I time, I want the disconnector to just kiss the hammer hook as the hammer is being cocked back on a untouched trigger.

The lower passes all the function test when off of the upper. Plenty of over lap between the time the disconnector has control of the hammer and when the trigger catches the hammer. The problem only shows when the upper is installed and cycled. With the upper on the disconnector  never retains  the hammer. The hammer follows the carrier back to the its fired postion even if the trigger is held fully pulled during the cycle.

This one takes some real thinking to solve not a common problem at all.

Since your getting hammer follow-threw, have you modified the top of the hammer to the point that the hammer is not lowered far enough by the carrier to catch the disconnector sear?

The problem is that the hammer is not going low enough but it is a factory hammer and so were the other 5 I tried. None were modified. The factory machined the trigger pin hole to low in the receiver. What suprised me was as low as it was it functioned in every other way. It is a little over 20/1000 too low as best I can measure it. I thought the carrier was off till I measured it. It took a while because of it passing the function test to figure it out. The lower is fairly new and from a major company. Only two post  great work.

You are using a M-16 hammer with the auto sear tab ground off?

If not, then try one to see if it corrects the problem.  This hammer will be lowered father than a L cut AR-15 hammer due to the higher upper edge/top, and may solve the problem.

If it doesn't then it time to send back the lower receiver for replacement.

Sending it back today.

Do you want to pass on the make of the receiver?

This may save some other guys a lot of headaches trouble shooting since most receivers are milled in batch lots, and chances are, there are others milled incorrectly just like yours.