M51 'Needler', M28 Launcher

The M51 Individual Combat Weapon System, with electronically integrated M28 Modular Munitions Launcher.
Like the M9 Javelin, this was a design for a story I was writing.
It was basically meant to be a rifle that followed similar principles to the MP7 and P90, with small, high velocity rounds for shredding armour. The under-barrel launcher was originally meant to be an anti-infantry weapon, but then I watched a certain anime and decide to add mechs to the story, so the launcher would also get armour-piercing explosive rounds for bringing those down. There is no trigger on the launcher because the rifle uses an electronic firing system and trigger, meaning the trigger can be switched to fire the launcher (note the "U" on the fire selector).
As for the list of users, the "Alliance Marines" was meant to be a close knit group of countries with a semi-integrated government, existing within and outside the overarching UN banner with the rest of the world.