If you look on the Jita market for projectile Ammo, one thing really jumps out as odd.... EMP is by far the most popular ammo type. Most days twice as much Republic Fleet (RF) EMP ammo is bought and sold than RF Phased Plasma and RF Fusion combined. This does not really make logical sense. It did make sense before the Dominion Expansion last fall. Back then RF EMP did more damage any of the other ammo types. However with Dominion that changed. Here is the new base damage for short range projectile ammo:
* Short Range (0.5 range modifier - 12 damage)
o EMP - 9 em / 2 explosive / 1 kinetic
o Phased Plasma - 10 thermal / 2 kinetic
o Fusion - 10 explosive / 2 kinetic
Ever since last fall all three types of short range ammo now have the same range and deal the same amount of damage. The only difference is the damage type. In general I always carry all three types of ammo and try to tailor what I am shooting to what I am shooting at. If I am going up against an armour tanked ship (say a taranis) then I load up the fusion, if I am going up against a shield tanked ship (say a Drake) then I load up the EMP. Otherwise I use Phased Plasma since it does good damage to both shield and armour.
So don't just load up EMP because that is what you are used to... try out the other projectile short range ammo, especially against armour tanked ships you won't be sorry.