In virtually all shooting games that use ammo, there is also a maximum ammo limit, which often varies between different ammo types. One obvious advantage of using ammo limits is to control the scarcity of ammo, so that players cannot excessively hoard it and drastically affect the game's difficulty.
What factors affect the size of ammo limits? An obvious factor would be the rate of consumption; if an ammo type is used by rapid fire weapons, the limit for this ammo type should be higher. But it should be obvious that there are many other factors, since some types of ammo quickly run out and others never seem to. As a designer, what factors should guide my decision for ammo limits, and when/why would I deliberately constrain or expand an ammo type's limit?