May I commend to your attention pgs 123-133 of Ezell's "The Black Rifle," where the "premature" or "indadvertent" firing problem with the M16 is discussed at some length. The solution taken was one of the "Eleven Pre-Production Modifications" on pages 135-136, to wit:
"5. Firing pin redesign (fig. 138)
"Finally incoporated as part of the TCC's "field fix" for the inadvertent fire problem, in conjuction with the adoption of less sensitive primers. After attempts by Colt's and Springfield to spring-load or otherwise partly restrain the firing pin, it was finally agreed to use a Colt idea which simply lightened the firing pin's total mass, thereby decreasing the force of its "bounce" off the primer." (emphasis added).
Figure 138: Firing Pin Redesign
On page 148 Stoner is quoted as saying: "Either desensitize the primer, make it out of a thick material, or lighten up the firing pin...I recommend...lightening up the firing pin because...if you desensitized the primer too much it could cause failures to fire in the field."
Yes, the locking lugs should be engaged for the firing pin to reach the primer, much as in the Garand-type action (M1, M14). However, a "Slam Fire" is simply the condition when a round is discharged without the normal firing mechanism being employed. Slam Fires may be caused by a broken/protruding firing pin, foreign matter on the bolt face, firing pin intertia, or other means. A Slam Fire may occur *in* or *out of* battery. In the first case, we have a truly Accidental (in contrast to a Negligent) Discharge with little or no potential harm to the shooter. In the latter, the breech is unlocked and *very* bad things can happen.
To be truly pedantic, there are those firearms that depend on "slam fire" for functioning, typically subguns such as the M1A1 Thompson. The firing pin is a bump on the bolt face.
In conclusion, where M1s, M14s and M16s are concerned, lighter firing pins are better, and use Mil-Spec primers. Hey, we spent a lot of tax dollars finding this stuff out!
For more information on Slam Fires, go to:
Slam Fire: A ParableSlam Fires, Mags & SLEDs; Clint speaks out!