In their new modular assault rifle produced by Zastava. The rifle can change calibers by switching out barrels, accomplished in under 60 seconds, back to the popular and standard 7.62 x 39. This is probably a great choice for flexibility since the Grendel was developed from the .220 Russian case, which in turn has its ancestry in the 7.62 x 39.

The platform is an AK system.

https://special-ops.org/m17-modular-assault-rifle-ar-zastava-arms/

The Grendel was seriously evaluated by several special missions units, they liked the round.

It was not adopted due to the very thin perimeter of the bolt face in an AR. They didn't feel the bolts would hold up to the volume of fire they would be running through each gun in training and in the field.

Not an issue in an AK pattern rifle...