Schematic: Core Marksman Rifle

Source
This item drops from about half the bosses in Molten Core:


 * Golemagg the Incinerator (2.6% drop rate)
 * Magmadar (1.7%)
 * Shazzrah (1.7%)
 * Garr (1.0%)
 * Baron Geddon (0.9%)
 * Lucifron (0.7%)