GP = General Purpose. The standard thing.
HC or LC = High Capacity / Less Casing (which esseantially is the same thing). Does only 75% damage a GP bomb of same size would do, but at 125% damage radius.
SAP = Semi Armor Piercing. 125% damage for only 33% of damage radius.
AP = Armor Piercing. 150% damage for only 25% of damage radius
Keep in mind bombs make much more bang for pound than larger ones. As a reference (GP bombs):
100lb - 156lb of damage
250lb = 313lb of damage (= damage rating of a town or factory building)
500lb = 550lb of damage
1000lb = 1000lb of damage
2000lb = 1718lb of damage
4000lb = 3125lb of damage (only theoretical, the 4k bombs in game are all HC/LC)
All damage rating for objects (as can always be seen in options/arena setup/object settings) are given in 1000lb.
Due to the current bomb modeling, which simply adjusts flat damage/radius instead of modeling armor penetration, it rarely (if ever) makes sense to chose one of the few planes with an SAP/AP loadout option over another with pure HE loadout. (for LW MA purposes)
For example the G4M with 800kg AP still has not enough capacity to sink an undamaged CV, while a lot of other bombers easily do.
Or for CV planes, the SBD with 1600lb AP has still less punch than a fully loaded F4U-D.
