The British in WWI produced a grenade cup that would fit on a 1 Mark 3 rifle by the simple method of turning a 2 and a half inch cup until a screw with clasps tightened up on the  muzzle of the rifle.  The device launched a special Mills Bomb grenade with a disk shaped butt end.  With a blank cartridge the rifle, butt firmly planted on the ground, could launch such a grenade between 80 and 200 yards.  The distance was controlled by angle of barrel and by moving a gas release at the base of the cup.  

Here's a good video produced for an auction several years ago. Now days the launcher cups go for around $150, although they were thrown in for free if you bought a wire wrap rifle once upon a time.  

https://www.youtube.com/watch?v=CT1DyyCxf9Q

That is pretty wild.  Thanks for sharing!

 

-Ski

And they are used on the Jawa blaster from Star Wars...