The discovery Explosive Missile says
As a standard action, the alchemist can infuse a single arrow, crossbow bolt, or one-handed firearm bullet with the power of his bomb, load the ammunition, and shoot the ranged weapon. He must be proficient with the weapon in order to accomplish this. When the infused ammunition hits its target, it deals damage normally and detonates as if the alchemist had thrown the bomb at the target.
The word can applies to all three of infuse, load, and shoot, which collectively take but a single standard action.
It's a matter of character and campaign whether this discovery is valuable; that is, to use it the character must A) usually make a ranged attack instead of a ranged touch attack, and B) factor in the additional expense, inconvenience, and weight of the weapon and its ammunition.
The discovery Explosive Missile is remarkably evenhanded. The discovery changes the typical crossbow-toting alchemist's bombs from bombs that're ranged touch attacks to bolts that're ranged attacks that also inflict bomb damage. Further, the discovery permits the alchemist to quickly load and fire that crossbow. The game realized the alchemist was already sacrificing a significant advantage by having the alchemist make a ranged attack instead of a ranged touch attack and compensated for that by permitting the alchemist--in some unexplained, maybe magical way--to reload and fire even a heavy crossbow as a standard action if the alchemist expends one of his bombs per day. That's a superior balancing act.