The sentence 微笑み選ぼう is part of the lyrics of a song called 錆びついたマシンガンで今を撃ち抜こう.
I'm wondering why it is correct in grammar because it seems to me that there is supposed to be a を between 微笑み and 選ぼう.
== Update ==
Here is the shorter version of the song for interpreting the meaning of this sentence: link.