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Old 09-16-2020, 9:28 PM
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ar15barrels ar15barrels is offline
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Originally Posted by slamfire1 View Post
I am of the opinion the most probable cause of your KB is bolt/carrier bounce and a sensitive primer.
The round is fed from the magazine (or single loaded in the chamber) the bolt/carrier closes, which allows the free floating firing pin to protrude out of the bolt face.
The firing pin then hits the primer, and bolt carrier bounce rotate the lugs out of engagement.
If you look at the AR-15 action in high speed, (several high speed videos concerning bolt bounce are available) you will see that the bolt bounce is FAR short of even the BEGINNING of bolt rotation, let alone enough bolt rotation to actually unlock the lugs.

Bolt bounce is usually under 1/8" to 3/16" while it takes over 3/16" of carrier travel to even BEGIN unlocking the bolt and around 5/8" total carrier travel before the bolt lugs would be unlocked and the cam pin cleared of the reciever enough for the bolt head to move rearward.
Even in this most extreme scenario with all the weights removed from the buffer, look at the carrier bounce that gets to perhaps 5/16" which is about HALF of the bounce that would be required to unlock the bolt from the extension:

Let's add one more fact into the equation:
The OP's gun is a rifle gassed 20" barrel which has a slower cycling carrier and therefore much less bounce than a midlength or carbine gas system.

I find your opinion not to be probable in the OP's closed breech high pressure event.
Randall Rausch

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