Obtain 200 Dark Iron Bullets.
Those gorge-digging dwarves do at least one thing right: they make very high-quality bullets. I've been unable to reproduce their results, so I've taken to stealing whatever bullets of theirs that I can find.
Ye want my help? Get me a couple hundred bullets from those marksmen below.
I'm sure they'll be happy to give ye their bullets one at a time, but you might find it faster just to look for the crates filled with them.