Improvised rifle grenades can be fired from most rifles with the right adapter, though adding a Hard DM to their hit roll. Their damage profile depends entirely on the grenade being shot.
Improvised rifle grenades consist of a Grenade 💣 that has been hastily attached to the end of a rifle barrel, perhaps supported by some kind of cup, then fired with a blank cartridge. This requires the user to pull the pin before firing, limiting their aiming time to a couple of seconds at most. Additionally, hand grenades aren't really aerodynamic and struggle flying even a few metres in direct fire. Overall a very unsatisfactory arrangement, but in the early stages of World War I it's simply what soldiers had at hand.