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Message: <blockquote data-quote="uncle money bags" data-source="post: 2166492" data-attributes="member: 8377">Both the 9mm para and 9x18 head space on the case mouth.I borrowed this from another forum.  When the cartridge is fired, before the bullet starts to move (remember the law of inertia - whatever is at rest tends to stay at rest, and the more mass it has, the harder it is to put into motion) the lighter brass case first expands out until it hits the chamber wall, and then it is slammed backwards until it hits the bolt (or, in the case of a revolver, the recoil shield). Once it has done that, the case can't move anymore, and the only thing that can move is the bullet, so it, finally, starts to move forward. If the distance between the head and the boltface (or recoil shield) is too great (this is referred to as excessive headspace) the case has farther to go before it stops, which allows it to build up more speed and more pressure before it hits, which can cause damage to the gun. Also, if the headspace is excessive, the firing pin may not be long enough to pop the primer, and you have misfires.In short, its not a good idea. Will it damage your gun or hurt you? That is somewhere between possible and probable, ymmv.</blockquote>