Some rifles are prone to unwanted ignition upon the bolt closing if the primer is incorrectly seated; some folks apparently prefer to buy an extra hard primer rather than learn how to seat their primers consistently at .oo3" under flush.......or ensure that the bolt is in good repair on the bargain basement M1they bought at the gun show.
I disagree with the overall impression that primer sensitivity is not an issue or that slamfires in mechanisms with free floating firing pins can be alleviated by simply seating the primer .003”
I found in an old American Rifleman that Frankfort Arsenal seated their Garand ammunition primers from 0.0” to -0.005”.
Primer sensitivity can be varied by crystal technology and of course the mix of ingredients in the primer.
For a mechanism with a free floating firing pin the sensitivity of the primer is matched to the impact energy of the firing pin on the primer and to the total energy of the firing mechanism. If the primer is too sensitive then a certain percentage of free floating firing pin impacts will cause ignition. If the primer is too insensitive then some firing mechanisms won’t provide enough energy for 100% ignition.