Any fan of shooter games knows the frustration of looting an enemy only to find ammo you don't want. Perhaps you prefer a shotgun and you keep finding machine gun ammo. Or perhaps you love the sniper and you keep finding grenades.
Sometimes it feels like the game is plotting against you. "No, use this weapon!" it says. The game keeps trying to coax you into a playstyle you don't prefer. Of course, this is often just the player's wishful misperception.
In any case, it raises the question of how ammo drops can and should be handled. I don't think there's any best answer -- what's good depends on the specific game and the designer's goals. But let's explore some of the options.
(1) Keep the drop frequency of all ammo even. This approach can encourage players to mix it up by ensuring they run out of their preferred ammo. How scarce any ammo type is depends on the number of weapons in the game. The more potential weapons in an area, the more likely the ammo found won't be preferred.
(2) Increase the frequency of ammo for the weapon the player is currently using. This approach encourages the player to pick a favorite weapon, to choose a playstyle. If ammo drops in general are high enough, it doesn't prevent players from switching weapons according to whim or changing scenarios (such as environment).
(3) Frequency of ammo drops are relative to each particular weapon's cumulative use. This accounts for players having preferred weapon sets, rather than preferences for a single weapon at a time. If a player likes to switch between a shotgun and sniper rifle, then more ammo is found for those weapons than for other weapons. This also means that if the player is carrying a sniper but hasn't used it in a while, he won't get extra sniper ammo he doesn't need.
(4) Encourage specific weapons for specific encounters through ammo. For example, if a particularly tough boss is more susceptible to incendiary rounds, then that ammo type drops within the boss's vicinity. In subtlest for, this can be used as hints to the player.
(5) Offer multiple weapons per ammo type. This encourages players to select a preferred genre of weapons, like in Medal of Honor: Airborne (rifles use one ammo, machine guns another), or a preferred ammo style (anti-personnel, anti-armor, incendiary, etc).
What other options are there? Which do you prefer?