From what I can see from looking at the weapon raws,(I haven't really tried raw modding in this release much) the reason most weapons have both edge and blunt, is because you can hit them with both the edge and the flat of the blade, or the handle, pommel, shaft, ect. If an arrow or bolt is using blunt, something is wrong, as you just shot the thing sideways. I feel the some holds true for this disc shooter. In fact, it looks like you only get one kind of damage per attack, so having ammo do two types of damage means it has two different kinds of attacks. One with an edge that rips into the flesh, and one that is the flat side hitting it, which I don't see being very effective, nor does it seem to be the feel the OP was going for. Of course, all this is just me looking at the raws. I haven't tested any of this out, as I'm a lazy, lazy man.
If I'm picturing it right, the ammo is basically a large buzzsaw blade, like what you might find Gorden Freeman tossing around with the gravity gun. In fact, that's exactly how I picture this weapon, minus the ability to shoot anything else, which doesn't make it any less cool.
Also edge represents both piercing and slashing weapons, depending on the size of the contact area, and penetration rating. For instance, the battle axe has a lot of surface area it hits with, but it doesn't go very deep, (unless it happens to, you know, chop off a limb) whereas the spear only has a contact area of 20, (less then bolts and arrows) but it goes almost twice as deep as an axe.
For some reason, I feel really dirty after writing that last paragraph. I'm gonna go take a shower.