The Republic of Korea Army has an OICW. In service.
It’s just like the XM29 OICW, which was supposed to be the future of the US Army soldier. The OICW fired a 5.56mm round like any other NATO rifle. The big gimmick was that in addition to that, it had a built in 20mm airburst grenade launcher with a fancy fire control computer.
That’s the XM29. It never caught on because frankly, it’s huge. It was also really complicated and didn’t work well enough to justify the costs.
Back when this program was going on, this gun was the future. An assault rifle combined with something that shot fully automatic explosive 20mm rounds. It sounds like something out of Warhammer 40k. I remember when every single video game was parading this thing around as the ultimate weapon and then it never happened.
The new Korean gun, called the K11, looks to be much more compact and simpler than the XM29 did. For starters, the grenade launcher is not automatic, it’s actually bolt action. Apparently it’s had a shaky introduction with some mechanically unreliable parts, but still, it’s truly the first of it’s kind to ever enter service.