I have been collecting links on Halabjah, probably more than you care to open:
http://home.columbus.rr.com/lfairban/Pages/Iraq.html#HalabjahA couple of things I remember, I don't believe it was artillery, but aerial, helicopters or planes. Also, there was a Pentagon study around '91 or so that concluded that the gas was dropped by Iran, not Iraq. I don't believe that conclusion, as it was probably an attempt to vindicate Iraq by people who had an interest of some sort in normalizing relations.
Either way, as a friend of mine said, we sold them the helicopters to do it with.
The biggest problem I have to people referring to Saddam Hussein "gassing his own people" is that they omit several key points: the Kurds are not his people as he is Sunni, they are only in the same country as delineated by the British and French at the end of WWI, there was a war going on at the time between Iraq and Iran and the Kurds were allied with Iran, and the Kurds have been trying to break off a chunk of northern Iraq and southern Turkey for decades so they can have their own country. This makes them civilians associated with enemy combatants, and their deaths are not much different than all the Iraqi civilians we killed when we invaded.