The M26 series is in many ways an upgrade in basic principle of the Mk II. A similarly shaped, but not visibly ribbed fragmentation grenade. The M26 has a filling of Composition B contained within a sheet steel two part outer shell which covers a pre-notched fragmentation coil inner liner. The use of the inner liner creates a highly predictable fragment pattern and causality radius. The grenade was found to have problems with complete detonation of the filler, and the M26A1 featured a tetryl booster to ensure complete detonation.