My guess is, that at present, the gun fires ordinary "dumb" projectiles. It fires metal slugs like our great-great-great ancestors used on their old wooden ships

. With a ballistic trajectory, this makes for hitting a speedy missile neigh impossible.
http://en.wikipedia.org/wiki/Otobreda_76_mm gives you a look at the oto-breda 76mm naval gun used on a lot of naval vessels.
As you can see, it can carry a meriad of different types of ammo:
- HE
- AP
- Training (lead-slugs)
- Proximity fuses / Timed fuzes / impact fuzes
- Incendiary
- AA
From what I recall from my naval years is that types of fuzes can be switched to match the task at hand. I believe this is done manually and not common practice.
For the 76mm in DW I'd love to see AA / Proximty rounds/fuzes so you can effectively engage airborne targets.
Proximity fuzes ->
http://en.wikipedia.org/wiki/Proximity_fuze