I had a PC kill an NPC with a fire extinguisher once. No, really...
The set-up had the PCs standing overnight 'cold iron' watches aboard a damaged subsidized freighter while it waited for repairs. Nearly everything was shutdown and shore power was feeding the gravitic nets, lights, life support, etc. Local port regulations required a 'rover' and bridge watch be aboard at all times and, because the subbie's crew had been transferred off, the positions were being filled from the local hiring hall. Enter the PCs.
The trouble came from two factors; the world was a vacuum world and a local group wanted to recover a package left aboard the subbie by one of the crewmen.
First, a couple of locals approached the watchstanding PCs during the day and tried to arrange access to the freighter. The PCs said no, reported the offer to the port, and recieved the usual bland assurances.
Next, the PCs on watch foiled and/or scared off an attempt by a local to sneak aboard and search for the package. Again they reported this to the port, recieved the same bored responses, and took matters into their own hands. The next night they brought their entire party aboard!
A group of locals came aboard that night with the intention of rounding up the watchstanders, locking them in a cupboard somewhere, and searching for the package. The wheels fell off when one of their number tried to collar one of the PCs on 'rover' duty.
The ship had atmosphere in it. The PC was wearing a vacc suit with his helmet slung. The local NPC was fully suited and had a snub pistol. When they saw each other, the NPC fired and missed. The PC immediately ducked into a stateroom. The NPC followed and the PC brained him with the only handy leavy object around; a fire extinguisher.
No GM intervention played a part. The PC rolled for and got a pinpoint hit on the NPC's helmet faceplate. The NPC was knocked down and the PC continued to whale on his helmet with the canister. After a few blows, I had to rule that the faceplate was cracked whereupon the PC jammed the nozzle against it and 'fired' the extinguisher.
From a planning viewpoint, the entire session went 'down hill' from there!
Have fun,
Bill