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 Question on the depth of firing pin indent '0.008 inch'
2byoung  [Member]
5/28/2010 1:27:09 AM EDT
I am looking for the origin of the depth of firing pin indent '0.008 inch' when, in a vertical, muzzle down position, the bolt carrier assembly is released from full recoil positon and the firing mechanism is not actuated (in Mil. Spec. of M16 or M4)
If firing pin indent is over 0.008 inch, is it possible for the primer to be detonated with 50% possibility?
I guess the value of '0.008 inch' was given to prevent slam fire when the bolt come into chamber and may be related to 'primer' and 'headspace'
But I can not find why the value became 0.008 inch in case of M16 and M4.
In addition, I want to know the releation of the depth of firing pin indent in case of using live ammo and copper pressure cylinder.

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Shermantor-AR15  [Member]
5/28/2010 4:56:59 AM EDT
Might get better answers in the reloading forum, when it comes to depths of firing pin penetration to set off a primer.

The firing pin on the M16/AR15 is free float and does not stick out of the bolt. However when you release the bolt it slams with force and the pin will sometimes make a small dimple on the firing pin and will not set it off, Also that is why for the M16/AR15, ammo is usually loaded with thicker primer cups to prevent this from happening.

The term slam fire, is probably different in what you are thinking, it's where there would be hammer follow which causes the firing pin to extrude out of the bolt to fire the rifle, now this does have the possibility of setting off the round because the hammer is under spring tension and may cause the firing pin to reach the correct depth or striking force to set off the primer. The ATF will use "soft" primer reloaded ammo to cause a slam fire effect from hammer follow.

So there are two basic situations here.
1) bolt is let go foward with large force but the pin is free floated and has a very low chance of firing the round, chances of going off are probably the same as being hit by lightning or even wining the lotto.

2) Bolt is going foward with the hammer following it putting pressure on the firing pin, which has a high probability of firing the round.

Dano523  [Team Member]
5/28/2010 9:13:04 PM EDT
FP in spec protrusion is .032 to .037 when the FP is maxed against the back of the bolt.

Now on the free floating FP, it is kept from protruding out of the bolt face via the back of the carrier until the bolt is fully locked up (cam'd home).

So as the round is strip and the bolt slams into the barrel extension, the FP will still not protrude until the bolt twists home. This twisting requirement is where a great deal of momentum is lost (hence the FP slamming into the back of the carrier, not the back of the bolt. And even better, if the hammer is riding the FP or B/C back home, there is not enough force for the slam fire as well (with all but the softest of primers not designed for rifles).

To get a true slam fire, the tip of the FP has to snap off and be frozen forward in the bolt. If that happens, then the round ignition can happen (think tommy machine gun) with the bolt still unlocked, and your going to be wearing most of the upper receiver in your face as the B/C comes back with 50K of pressure driving it back uncontrolled.
2byoung  [Member]
5/29/2010 1:49:49 AM EDT
Thank you for your answer.
But, I want to know why the the depth of firing pin indent (bolt action indent) is limited to 0.008 inch in case of M16/M4.
Is there anyone who knows the reason?
Stahlgewehr762  [Member]
5/29/2010 11:42:11 AM EDT
Originally Posted By 2byoung:
Thank you for your answer.
But, I want to know why the the depth of firing pin indent (bolt action indent) is limited to 0.008 inch in case of M16/M4.
Is there anyone who knows the reason?

Because a firing pin indent much deeper than .008" would be getting dangerously close to setting off the primer and blowing up the rifle would be my guess. Ideally, there would be NO firing pin indent when the bolt and carrier slam home on a live round. However, due to the free-floating firing pin of the AR-15/M16, there will be some degree of firing pin indent due to the inertia of the firing pin hitting the primer when the action closes.

This is a case of "the less there is, the better". The more shallow the firing pin indent, the larger the safety margin between you and a blown-up rifle. The nature of engineering is to put a specification onto everything, and apparently .008" was determined to be the maximum safe limit for this parameter.
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