A bit less than real life shells of roughly the same caliber, I'm lowballing really hard here, given that I don't have an exact volume for the round and am instead using the volume of a box it can fit inside, but that calculation yields about 1.5 g/cm cubed. Of course, that's a moot point given the fact that the round in question is moving about 327761% faster than this .50cal.
If you want to keep this weapon at the tier it's at now, you should either increase the size of the round this Daedelus cannon fires, increase the velocity, or preferably; do both.
Alternatively, we can estimate how much damage this would do based off of existing weapons, though that's definitely going to mean it's damage is reduced a large amount.