Ruins by Dan Wells. This is book 3 in The Partials series.
Here is the description from Amazon:
Kira, Samm, and Marcus fight to prevent a final war between Partials
and humans in the gripping final installment in the Partials Sequence, a
series that combines the thrilling action of The Hunger Games with the provocative themes of Blade Runner and The Stand.
is no avoiding it—the war to decide the fate of both humans and
Partials is at hand. Both sides hold in their possession a weapon that
could destroy the other, and Kira Walker has precious little time to
prevent that from happening. She has one chance to save both species and
the world with them, but it will only come at great personal cost.
I am always a sucker for a good apocalypse story. I like it because it is original and good development of the characters.
--I wish authors would do better with gun details though.