I'm sure it's been done many times in many variations over the spell's history, but I'm having trouble finding any reasonably conclusive ruling or argument as to whether or not it should work. Is it possible to use shrink item on a large, dense object, then fire the tiny version of it and un-shrink the projectile mid-flight?
The first obvious method is to activate the spell's command word before it strikes its target, thereby upsizing it and retaining its momentum. A command word is generally a standard action, so this clearly won't work in close-range scenarios, but would it be possible at long range where the projectile conceivably took more than one round to reach its target? Does the item have to be at hand for the command word to work? If not, what's the range of the command word?
The second method would be to simply allow the shrunken item to resize automatically after striking its target. Would it retain any momentum as it increased in size? Would the expansion be fast enough to deal damage on its own?