40 Blasting Damage in one hit? It wasn't the explosion that proc'd (those don't even appear in the log).
I just checked Dredmorpedia, and the Clockwork Drill Bomb creates an effect "Ticking Time Bomb", which gives -10 to blasting resist for 4 turns, which can stack up to 5 times. So that damage is PROBABLY correct.
I don't know, the hit being displayed in the log is a one-hit-kill, and neither my ranged attacks nor the initial hit of the arrow deal blasting damage. I figured it had to be related, as the connection between blasting damage and the bolt is obvious, but I have no idea where it is coming from.