On the other hand, we recently had a member who did have a slam-fire as a result of chambering commercial ammo in his rifle.  While extremely rare, it CAN and HAS happened, and is certainly a possibility with any rifle that has a free-floating firing pin (i.e., most military designs).  Always keep this in mind when chambering ammo at home or anywhere but the range, and maintain muzzle discipline.

Another thing to be careful of is re-chambering the same round multiple times.  The military and cops both do this frequently, as they have load their rifles at the beginning of a shift and unload at the end.  Rechambering the same round several times can either lead to a slam-fire, or a primer ignition failure.

Finally, keep in mind that commercial ammo does not use the less sensitive military primers, and are more likely to have problems than military ammo.

-Troy

Very few military guns have a firing pin return spring.  Relying on one in the design would make the rifle dangerous to use without it (i.e., lost during field-strip), and would force heavier springs and parts to be used to insure primer ignition, which would make the trigger pull heavier, etc.  Guns are systems, and changing one component can have a chain-reaction effect.  Note that a firing pin return spring was actually tried at one point, and rejected.

-Troy