If you hold a bow in your hand while right clicking on a minecart command block, the GUI will open and the bow will charge. Upon exiting, the arrow will fire, most likely shooting and destroying the minecart.

This also applies to:

• Splash potions
• Lingering potions
• Eggs
• Snowballs
• Water buckets
• Lava buckets
• Ender pearl
• Eye of ender
• Bucket of milk
• Botte o' enchanting
• Fishing rod
• Shield