The AK-15 is a variant of the Russian AK-12 chambered in 7.62x39mm. It was designed to be a successor to the AK-74m in service with the Russian Army.
The model itself is based on the early / prototype versions of the AK-12 also known as the AK-200 because I like how the blocky top cover and handguard looks.
The RPK version is not based on the real world light support RPK-16 variant of the AK-12 but is instead a combination of the barrel and bipod of the RPK-74 with the body of the AK-15, again because I like how it looks.
The grenade launcher is based on the GP-25 underbarrel grenade launcher