I don't think you will damage your gun or choke shooting buckshot thru it, but my gut is that you would be way over choked with 00 buck thru a .665.
If you don't want to buy an additional choke, I would suggest you try Federal Premium WITH Flight Contol Wad out of you improved cylinder choke. Your best bet would be to buy a choke optimized for buckshot. I have had great success with Comp-n-Choke Deer choke. They have two different choke constrictions depending on what size buckshot you shoot. If you select this choke, try the Federal Premiums WITHOUT the Flight Control Wad.