It is a critical dimension for several reasons, but there is a fair amount of leeway.
Obviously, a firing pin that doesn't protrude enough will result in misfires. One that protrudes too much will get in the way of the cartridge when feeding from the magazine and jam the rifle. And then if the round does ride up the bolt face, or a round is fed singly, chambering it with a protruding firing pin can set off the primer before the bolt is locked, with no real good result.
So a firing pin that will protrude too much only when the bolt is closed and locked is unlikely. There are many warnings that a excessive firing pin protrusion or an over-strength firing pin spring will cause "pierced" primers, but that is not true. The oddity is that "pierced" primers result from a too weak spring or a too light firing pin; but often folks who experience that condition take bad advice and weaken the spring further, which results in more primer "piercing", so they weaken the spring more, with more "piercing", until the gun doesn't fire.
FWIW, I have deliberately used a firing pin with a very long nose and struck it with a hammer to drive it into the primer. I could never get it to "pierce" a primer or the primer metal even when it crushed the anvil.
Jim
