[Rabbi]

After many years of reloading, I still don't know exactly what causes primer setback. Is there one reason in particular? I had a firing pin spring failure yesterday at a combat shoot simulation and when I changed out my spring last night, I tested the new one on some primed only cases.

The primers jumped out of the pocket about .045" and were deeply indented. Am I using too weak a spring? Too strong a spring? Can setback happen with a primer only case and might not with loaded ammo? I won't get to live fire the pistol for another week or so.

Thank you in advance.

Doc

edited/ not mag spring, firing pin spring.

[Old Soldier]

It could be that the primer pockets are getting to be a little over size, or the head space needs to be checked.

[Loader 3009]

DocWilly, this will happen in both revolvers and autos, the reason being that the revolver recoil shield or in the case of the auto, the face of the slide re-seats the primer when using loaded ammo. In your case, the casing (the brass) did not "set back" out of the chamber and re-seat seat the primer. In other words a fully loaded case, when fired, does not "grip" the chamber, it recoils until it strikes the recoil shield or face of the slide.

In the case of a revolver, it will lock up the cylinder. If this distance is excessive, it will affect accuracy,

[Dutch Henry]

Loader 3009 is correct. I've experienced the same thing when testing new springs and firing pins; the primers will back out of the case, for the reasons given.