Like many superhero narratives, Arrow has issues with guns. Guns are almost exclusively used by the bad guys, but they'll selectively forget to use them at crucial times. We see that during the same season 4 finale: at first, Damien Darhk has a legion of armed HIVE troops at his command. Later, however, we have a major plot point (majorly awful, that is) that Oliver must channel the powers of positivity and hope in order to defeat the dark energies of Damien Darhk. No, I'm serious: Oliver handles his Big Bad the same way Care Bears and Ghostbusters sequels handle their problems.
In order to represent that, we see him inspiring his rioting city by standing on top of a taxi and speechifying. It works, and these motivated citizens end up taking back their own city when they fight Darhk's HIVE troops. These troops are apparently so eager to engage in fisticuffs that they don't just…well…shoot the citizens with their guns. The same citizens they're already trying to kill via nuclear apocalypse. It's utterly bizarre, but par for the same show that once established Oliver Queen can use guns in a non-lethal way with precision accuracy, yet he chooses to use a big, awkward weapon that cannot be easily reloaded. In all fairness, though, "The Green Gun" sounds more like a Lex Luthor art exhibition than a superhero, so Oliver probably made the right call.