When unconfined ammo ignites it is generally a pretty feeble afair as the the pressure does not build after the projectile and case are pushed apart - there is simply no barrel for the powder to burn inside. Since the case is often lighter than the projectile it is usually the case which moves the most with the projectile barely moving at all. Both have so little energy imparted to them that a couple of layers of strong cardboard usually stop them.
I have personally witnessed tests of this happening.
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