Reproduceable Bug with the DYAD CROSSBOW.

Act 2.
Get a basic DYAD CROSSBOW.
Equip it
Enter a dungeon instance.
Fire two shots like "Frag Rounds"
Swap it to your second weapon tab and use your main crossbow with the additional bolt.

I ran across this, because I didn't know really what the description of additional bolt meant. I moved it to my second weapon tab to save room. My main crossbow now has all of its benefits plus the Dyad additional bolt.