Talk:Weapons/@comment-2.220.23.200-20150406193756/@comment-26286858-20150417152444

You don't want to sell unused weapons. It would be in your best interest to level it to 30, and then sell it.

The main way to go up in Mastery Rank is by leveling up equipment(Warframes, weapons, archwings and companions) and you want to rank up your mastery, as some equipment and content is mastery-locked such as the Rhino Warframe, the Soma and the Dragon Nikana.