Milkor 40 mm UBGL grenade launcher
The Milkor 40mm UBGL grenade launcher is a lightweight single-shot, underslung grenade launcher designed and developed in South Africa by Milkor (Pty) Ltd. It can be attached to most modern assault weapons and rifles using a Picatinny rail system.
Source: Wikipedia — Milkor 40 mm UBGL grenade launcher (CC BY-SA 4.0)