From my years of pondering RPG combat mechanics, including many games with lots of missile use like the Robotech RPG... the odds of shooting down a missile without some sort of directed energy weapon controlled by an advanced computer tracking mechanism are pretty low as is.
Shooting a fragmenting projectile at a missile coming in could work, but also cause collateral damage.
Shooting a direct missile at another missile is just absurdly difficult.
Just speaking of game mechanics, which aren't real life of course, but it gives you an opportunity to really ponder the difficulties involved in a different way, and inspires lots of discussions among geeks, many of whom are or were in the military and have real experience with these systems to various degrees.
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