(I used Python Fitting Assistant to check these different fittings out)
Here are the stats that I got for different types of ammo (click here to view the full stats for small projectile ammo):
- Carbonized Lead S - 30.8 DPS
- Depleted Uranium S - 38.5 DPS
- EMP S - 50.1 DPS
- Fusion S - 46.3 DPS
- Nuclear S - 34.7 DPS
- Phased Plasma S - 46.3 DPS
- Proton S - 34.7 DPS
- Titanium Sabot S - 42.4 DPS
The DPS is for the turrets of the ship which includes a few other types of turrets and other enhancements. Your mileage may vary but I think the overall percentage improvement that each ammo type gives you stays the same.
I also have a single railgun, the 250mm Railgun I, and I changed the charges on that (it uses Medium-sized charges, click here to view the full stats for medium hybrid charges):
- Antimatter Charge M - 53.1 DPS
- Iridium Charge M - 45.5 DPS
- Iron Charge M - 42.4 DPS
- Lead Charge M - 47.0 DPS
- Plutonium Charge M - 51.6 DPS
- Thorium Charge M - 48.5 DPS
- Tungsten Charge M - 43.9 DPS
- Uranium Charge M - 50.0 DPS