Firing Out of Battery

Like MadDogDan stated: it is a mechanical impossibility to fire out-of-battery unless something else went wrong.

Take a look at the firing pin block on the underside of the 5-7's slide. See how it is angled? Hold the slide over the frame, and pull the trigger (thumb the hammer up first so you don't beat the frame). Note the little knobby thing on the right side of the hammer. The sole purpose of that lever is to unlock the firing pin as the trigger is pulled. It cannot do that until the slide is all the way forward. It will not allow the falling hammer to touch off the primer until everything is seated.

Now, with the slide off the frame, position the slide 1/4" to the rear. See anything? The lever that manipulates the firing pin block can't even touch the firing pin block that far out-of-battery. About 1/8" to the rear does the lever even touch the block, but again: the angled cut of the block means the firing pin stays put until the slide is closed.

Now reattach the slide. Pull it so it is 1/8" back. Check the barrel and slide. Notice anything now? Let the slide forward and pull it back 1/8" a few times. Check the barrel block through the ejection port. The barrel is fully seated against the breech face. The barrel is in battery from the 1/8" point forward, so even if the firing pin was released early, it wouldn't cause a catastrophic failure.

I'll say it again: mechanical impossibility.

The design prevents the firing pin from being unlocked until the weapon is in battery. Even if the hammer drops, it can't fire the pistol until all is locked up.

The kind of damage on that pistol can also be caused by an over-pressured round. Pressures too high can force the chamber open before the bullet passes and result in a case failure.

I placed little circles of scotch tape over my spent brass to smooth the surface out and make it easy to see contact.

At 1/4" out, click of hammer fall, no mark on the primer.
At 1/8" out, hammer fell, no mark.
Here's the kicker: The weapon was out of battery only slightly because the once-fired brass was slightly oversized and the chamber would not close completely. I measured about 1/32" out. Hammer fell, NO MARK ON THE PRIMER. I pounded the slide closed a second time, and got it close enough to where the firing pin finally made contact.

I am completely convinced this is a non-issue; that unless something else went wrong with the KB'ed pistol, there is no way it fired out-of-battery. I'm wholly convinced the failure came from the reloads and not the pistol.

Important note for using primed brass: The hammer is going to try and force the slide closed when it falls- don't let it. Drop the hammer, then extract the round. Check it for marks, don't rely on the primer firing alone. Wear ears and eyes, those little primers are loud. Keep the muzzle pointed in a safe direction as if the pistol was loaded with live ammo. If the rounds have not been resized, they will not fit in the chamber all the way.

You'll see this out-of-battery theory is utter nonsense.