Guns: Rifle Grenades
The rifle grenade is an interesting answer to a problem that was posed when grenades were first used in warfare. A grenade's range is obviously limited by how far you can throw one - and unless you have a few Nolan Ryans in your unit, there's a good chance that distance isn't very far. Of course, you might be able to use a grenade launcher to extend this range, but that means not being able to carry a rifle...
Enter the rifle grenade. It harnesses the propulsion and sighting system you already have - you stick the grenade on the end of your rifle, and voila! Your rifle just became a grenade launcher. It's a bit more complicated than that, though. Early rifle grenades didn't have a bullet trap system (to catch the bullet and gases as they exit the barrel), so you had to use special blanks (if you mistakenly tried using a live round with one of these on your muzzle...).
They've been mostly replaced by integrated underbarrel grenade launchers, but they still work: