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I'm trying to figure out how some weapons differ, more specifically when it comes to D weapons with both the Primary Weapon and Ordnance.

A specific example being:

Volcano Cannon: Destroyer, AP 2, Ordnance, Primary Weapon
vs.
Bursta Kannon: Destroyer, AP 2, Ordnance

Does Primary Weapon or Ordnance have any effect on the Destroyer Weapon Attack Table?

Are the two rules cumulative? (So Primary Weapon Ordnance would be 3d6 pick highest on the penetrating hit chart.)

Volcano Cannon is Primary Weapon 1, it doesn't have Ordnance anymore.

But I'm also curious to the interaction of Primary Weapon and Destroyer.

primary only lets you reroll pens (like ordnance). It does nothing for destroyer.

Primary weapon used to stop weapons getting blown up, so a lot of d weapons will still have it listed.