The Missile Launcher.
The Eldar Missile Launcher has that sort of tear drop shape in the main barrel.
An ideal option for this would be teardrop shaped beads.
http://www.ebay.com/itm/16-Inch-Strand-Czech-Glass-Teardrop-Shaped-Tear-Drop-Beads-in-Sizes-Big-to-Small-/190765383114
This is a UK eBay link, but I'm sure you find similar for quite cheap at a good number.
Of course you don;t get the grooves near the end of the barrel, but as a bead it'll have a single hole on both ends, one which could easily stand as a barrle and another to allow you to pin or magnetize the bead onto the body of the Dark Reaper's existing gun.