Under-barrel grenade launcher
From Wookieepedia, the Star Wars wiki.
The under-barrel grenade launcher, also known as an over-under, was a special forces weapon designed as a miniature rocket/grenade launcher. The weapon could be attached to a DC-15A, DC-17m, WESTAR-M5 blaster rifle or E-15A blaster rifle via an underbarrel attachment, and could be configured to fire a variety of other explosive projectiles.
Advanced Recon Commandos put these weapons to use when rescuing Jedi Master Ki-Adi-Mundi and several other Jedi from General Grievous on Hypori. Although the salvo of grenades attacks did not significantly harm the incredibly agile Separatist general, they did manage to keep the cyborg preoccupied evading the explosive barrage.
Mace Windu also briefly used this weapon during his mission to Haruun Kal.
