Identified the Problematic Cannon

Quest Text I hope things don't deteriorate to the point of using the cannons.
My friend joined the Rebels. Have you been to the Castle Ruins?

Summary Joel sent you?
This Cannon right next to me has issues.
You see the crack on the barrel?
It's obvious, but he keeps saying he doesn't see anything...
I was about to go to him directly myself.
Please inform Joel about this for me.