Yeah, you're thinking of the XM29 "Objective Individual Combat Weapon," which was an assault rifle with the 20mm programmable grenade launcher on top. H&K essentially removed the smart grenade system to create the XM8. They also made the XM25, which was just the smart grenade launcher bit. All of those weapons never got past prototype, though all of this research eventually produced the M320, which is a non-smart under-barrel 40mm grenade launcher.
There's also South Korea's K11, which is the same concept as the XM29, and seems to actually be used, though I can't find much info about its effectiveness.
The conclusions from the failure of the XM29 I believe was that it isn't trivial to operate a smart grenade launcher, and the 20mm grenade doesn't have as much lethality as you would like. But perhaps the K11 was able to rectify these issues. I personally think this need for precise, powerful fires could be filled by a crew-serviced weapon mounted on an APC/IFV. Doesn't even have to be a smart grenade, could be a special autocannon or something.