From Small Arms Defense Journal:

The genesis of the resulting HK121/MG5 from the native firm Heckler & Koch came with the German Bundeswehr’s (Armed Forces) “Infantry Soldier of the Future” program, a comprehensive modernization effort concurrent with similar initiatives in the U.S. and other NATO partners.

Striding past another FN design, the commonly fielded MINIMI which is best known as the M249 in U.S. service, HK‘s innovative 5.56mm gun won the competition for Germany’s SAW (squad automatic weapon) role, designated MG4 in logical nomenclature progression. Fielded in 2004 and soon in vicious firefights in Afghanistan, the MG4’s reported reliability, accuracy, soldier-friendliness, and modular adaptability provided additional validation. But with two notable exceptions:

More powerful 7.62 x 51mm NATO ammo was needed for the longer range engagements that characterized the fight against Taliban insurgents.