Analysts call the ATV a baby boomer. Nevertheless, it will carry the small range K-15 or the longer range Agni 3.
Information about the K-15 is classified, but DRDO released its land based cousin Shourya.
Some pictures,
The Shourya being a version of K-15 is being denied officially. But, analysts are not convinced.
You can expect the same to be the K-15 without the underwater booster. The K-15 will fly to an altitude of 5km with the underwater boosters and then the second stage kicks in.
Both, K-15 and Agni 3 are ballistic missiles.