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Durkin Tactical Franklin Armory
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Posted: 11/20/2021 10:50:02 AM EDT
Link Posted: 11/20/2021 12:29:08 PM EDT
[Last Edit: OTTOSEAR] [#1]
All of my Colt System bolts would reliably trip the auto sear on my NFA Sendra.

The hammer should drop around 1/8 of an inch IIRC. Test it by manually racking the bolt on an empty chamber. Selector on auto, with the trigger back.
Link Posted: 11/20/2021 1:25:25 PM EDT
Bolt bounce?  What buffer(s) have you tried?  In my experience, with mild bolt bounce a two or three round string before getting a light strike failure to fire is typical.  With more severe bolt bounce, only one round will fire.
Good luck.
MHO, YMMV, etc.  Be well.
Link Posted: 11/20/2021 5:30:46 PM EDT
[Last Edit: jbntex] [#3]
Most 9mm pattern AR15 bolts I have ever seen are full auto.

Do your uppers trip the sear and release the hammer when you manually cycle the action with the trigger depressed?  That will confirm if its a full auto bolt or not.

Assuming you have a bolt that is tripping the sear there are two other issues that need to be potentially resolved on a 9mm pattern M16.

1. Bolt bounce.  On the 9mm guns since they have no locked breach you really need a heavier buffer with a couple of the antibounce weights being tunsgten to keep that bolt closed and prevent bolt bounce.

2. Firing pin bounce.  The 9mm bolts have a spring loaded firing pin and the firing pin cannot reach the primer directly as it is an inertia style firing system where the hammer hit the firing pin, it launches forward, and inertia carries the firing pin into the primer to set off the round.   The spring around the firing pin is there to keep the firing pin at the rear of the bolt when it slams closed so that the bolt can squarely hit it.  (as well as provide protection from slam fire on soft pistol primers)

A lot of the "consumer" 9mm bolts have firing pin springs that are too weak to keep the pin securely at the rear of the bolt carrier during closure.  So what happens is the bolt slams home and the firing pin moves forward into the interior of the bolt, the hammer drops, and the firing pin is essentially floating between the primer and the rear of the bolt so the firing pin doesnt get a good strike by the hammer.    Usually replacing the firing pin spring with an OEM Colt spring will resolve the issue.
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