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From: (Bart Bobbitt)
Subject: Re: Primer Sensitivity
Organization: Hewlett-Packard Fort Collins Site

This is a common thing with both the M1 and M14/1A rifles with their
floating firing pin.  When the bolt slams home, the firing pin is free
to keep moving about a sixteenth of an inch, but is stopped by the
primer in a loaded round.  But it doesn't have enough energy to compress
the priming compound and detonate it.

A common belief is that Federal primers shouldn't be used in these
rifles.  The reasoning is that their primer cups are softer than most
others and that will allow the firing pin to go further into them and
detonate the primer.  I don't belive this for a minute.  As Federal
primers are used about 50 times as often as other brands in service
rifles for highpower competition, I think it's reasonable to believe that
all these `slam-fires' have a higher incidence rate with them than others.
In every situation where a slam-fire has occured that I've seen the
ejected case, its primer has the same size dimple as one that was fired
normally.  I have never seen a slam-fired case with a
dimple shallower than one caused by the normal indentation the floating
firing pin makes.  If a slam-fire is caused by the floating firing pin's
forward movement after the bolt closes, the resulting dimple MUST be
shallower than a normal dimpled case.  All that pressure will press the
primer cup back some amount; this, I've never seen in observing several
dozen cases from slam-fires.  Compare the dimple depth in a normally fired
primer with a fired case that's been chambered and dry fired; there's quite
a depth difference.  So, I'm convinced that virtually all slam-fires are
caused by insufficient sear and hammer hook engagement.  I've dry fired
several M1s and M14/1A rifles with so little engagement, they would
disengage when letting the bolt close on an empty chamber.  And I've
never seen or heard of a slam-fire happening when the safety's on; so
that strengthens my opinion.  But there's probably an exception to my
reasoning; that's happened before.

Federal primers do have softer cups than most other primers.  They are
also used by the top military highpower rifle teams in both handloads
and Federal factory match ammo (that shoots much better than the LC
match ammo they choose not to use for most matches).


From: (Bart Bobbitt)
Subject: Re: [AR-15 Accidental Firing]  Does it happen
Organization: Hewlett-Packard Fort Collins Site

Yes it does.

Virtually all `slam-fires' are caused by one of these situations:

  * The primer is not seated deep enough in the case.  As the bolt
    rapidly closes on the chambered round, the primer is pressed
    fully into the primer pocket very quickly.  That oft times will
    cause priming compound detonation which fires the round.

  * Sear engagement is insufficient.  When the bolt slams into
    battery, the resultant shock disengages the sear letting the
    firing pin be struck which fires the round.

  * Disconnector not functioning correctly.  After the bolt cycles
    from firing the previous round, the disconnector fails to retain
    the hammer back after the bolt closes on the next round.  The
    hammer lets go, then striks the firing pin.

  * A combination of too light a trigger pull, too little backlash
    and poor trigger finger control.  After firing one round, the
    trigger finger moves forward/backward in such a manner that the
    mechanics of doing so releases the sear immediately after the
    bolt's in battery firing the round.

Having seen dozens of slam-fired cases over the years from both
7.62mm and 5.56mm semiautos, 99% of 'em had primers dimpled exactly
the same as a normally fired round.  That's proof that the firing
pin struck the primer with normal, firing force; something caused
the hammer to strike the firing pin immediately after the bolt

3 or 4 slam-fired cases I've seen had no dimple to speak of in their
primers, but their primers were sticking out of the primer pocket
more than normally seated ones do when fired.  When compared to other
cases from the same lot that normally fired, the normal ones were
dimpled; those few had practically no dimple at all.  One of 'em had
a hole where the dimple should be; perhaps the pressure blew the
primer's center back into the bolt but that wasn't checked.  In two of
these situations, I inspected other loaded rounds from the handloaded
ammo.  A few of 'em had high primers; those primers stuck out about
.010-in. or thereabouts.


From: (Bart Bobbit)
Newsgroups: rec.guns
Subject: Re: [reload] Garand question
Date: 30 Sep 1994 14:52:19 -0400

Hey CoolBreeze! ( wrote:

: Are slam-fires
: as great a concern in the M1, as say in a SKS?  Should I use
: a harder primer?

Having seen several dozen of these so-called `slam-fired' cartridge
cases from several M1s and M14s and M1As, each and every one had
one common indicator; the primer had a complete dimple exactly like
the one that normally fired cases had.  Everybody else I've talked
with who have also seen slam-fired empty cases has made the same
observation; primers are dimpled normally.  I don't know of anyone
who has observed a slam-fired case that has no dimple in the primer.
All of which means these slam fires happened because something made
the firing pin drive hard into the chambered round's primer.

And I and others have observed many ejected live rounds from these
rifles with all makes and types of primers used in them.  The amount
of micro-dimpling caused by the floating firing pins bouncing off of
them as the bolt closed is so darned consistant that trying to 
identify whether or not the primer has harder or softer cup metal is
an exercise in futility.

Should one care to study the design of the bolt assembly in these
rifles, it will be noted that the firing pin has a dog-leg at its
back end that prevents it from being put in contact with the round
in the chamber until the bolt is almost 90% closed.  That dog-leg
has to pass through the slot in the receiver's safety bridge and it
won't do that until the bolt rotates closed.  If the safety bridge
or firing pin dog-leg is damaged, worn or otherwise not mechanically
correct, it is possible that the firing pin can make full contact
with the primer before the locking lugs are engaged enough for safe
operation.  There are gages made to check the safety bridge for
correct dimensions so a correctly dimensioned firing pin will not
touch the primer until the bolt is safely closed and locked.

Lets assume for a minute that the firing pin is wedged forward in
the bolt fairly tight, perhaps by grit or corrosion.  When the bolt
slams forward chambering a new round, the firing pin's dog-leg will
be full forward in the notch at the rear of the bolt.  When that
dog-leg slams against the receiver's safety bridge, one of three
things will happen:

  * The firing pin stays where it is freeing itself from whatever
    stuck it in place.  Then the bolt continues forward about another
    3/16ths of an inch, rotates and locks in battery.  The round does
    not fire as the bolt closes.

  * The dog-leg breaks off because the firing pin is both faulty and
    is wedged very tight in the bolt.  Depending on how the back end
    of the firing pin breaks, it may not be usable any more.  And the
    broken-off piece falls down into the action.  The chambered round
    may fire if the stuck firing pin stays in place solidly enough.

  * A piece of the receiver's safety bridge gets broken off by the
    firing pin's dog-leg.  That piece will fall into the action.  If
    the firing pin remains solidly wedged in the bolt, it may impact
    the chambered round's primer hard enough to fire the round.

Inspection of all the rifles I've known (and those others I've talked
to about this) that have slam fired have not had any of the above two
slam-firing situations occur; no parts were damaged and the safety 
bridges were dimensionally correct.  So, our conclusions are that the
firing pin was not stuck in the bolt nor were primers too soft.  There
simply was no evidence to support these two claims.

So, what did cause the slam-fire to happen?  Whatever caused these
several dozen slam fires to occur resulted in normally dimpled primers.
What causes normally dimpled primers?  The hammer striking the firing
pin.  Which strongly implies something caused the hammer hooks to
disengage from the sear as the bolt slammed into battery.  What causes
that to happen?  Two things: the trigger gets pulled as the bolt locks
into battery, or, the hammer-sear engagement surfaces are so slight
that they are vibrated apart as the bolt slams home; the bolt is closed
as the hammer falls which drives the firing pin into the primer leaving
a normal dimple in the ejected case's primer.

In conclusion, should a slam-fire happen with an M1 or M14 or M1A, 
inspect the fired case and the rifle's action.  If the primer has a
normal dimple in it, and there are no broken parts in the action, and
the firing pin and safety bridge are correctly dimensioned, the thing
that caused the slam-fire was the hammer falling on the firing pin.

One final question; how many folks have seen a slam-fired empty case
that did not have a normal, complete dimple in its primer?


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