Heckler & Koch G11.
This is one of those weapons that shows great promise, but is not going to become a mature technology, unless there is a major war.
The G11 uses caseless ammo, which offers a lot of theoretical advantages, such as being able to carry much more ammo for a given weight. Unfortunately, the rifle is currently several times more expensive than its conventional counterparts, while offering only small improvements on performance. Right now, no nation is willing to commit the funds necessary to fully develop this weapon and improve the cost/benefit ratio.
This was the fate of a lot of weapon systems - they were actually conceived prior to a big war, but only when the big war came were people pushed to actually spend enough to make the weapon practical. The jet fighter and the atom bomb come to mind.
I was just thinking of the same thing myself!