Many years ago, there was a grenade for the M-16 that looks like a bowling ball and look nearly as large. If I remember correctly it was for demolishing walls or something.
Anyone else remember this?
Rifle grenades were the original way of launching grenades with a rifle.
The M-79 (grandfather of M203) was a more recent development - Vietnam War.
Even then, some armies continued to develop rifle-launched grenades including - if I'm not wrong - the IDF with the Galils. And yes, you need to load a blank.
The preference for M203-type over a rifle-launched grenaded is obvious:
If after attaching a grenade to the rifle muzzle, the grenadier cannot shoot if an enemy soldier suddenly appear 2m near him. With a M203-type weapon, no problem. You can load the grenade, put M203 to safe, and continue engaging with 5.56.
THis is the reason why most armies switched over to the GL underslung like the M203.
The next reason is that having to load a blank round or change mags after you have already wasted time attaching a grenade to the muzzle is just too time-consuming. And the nightmare scenario is firing a live round into the grenade instead of a blank one.
Yes there are those where you don't have to change ammo. But by the time these were developed most armies have lost interest.