A 5 inch cast iron rifle grenade on a 12 inch iron rod.

The rifle grenade No. 20 Mk I, dated June 1917, is a simplification of the rifle grenade No. 3, making it more economic and simple to manufacture, without sacrificing anything on the safety side. Both the externally prefragmented steel body and the internal percussion detonator mechanism remain unchanged, but the safety wind vane is replaced by a simple sliding releasing socket pulled back by the departure momentum, blocked at rest by a safety pin.

A further simplification of the steel body machining was introduced later with the rifle grenade no.20 Mk II, whose prefragmented body was only having parallel circular machined grooves instead of the classical square pattern.

As all these kinds of projectiles, this rifle grenade was used by inserting the tail rod inside a usual infantry rifle gun, but armed with a blank cartridge bullet-free. The accidental use of a conventional bullet was the main cause of incidents.