The primary weapon of Nepleslian power armors is this:
https://wiki.stararmy.com/doku.php?id=nepleslia:hpar . Because power armors are essentially walking tanks, they need to carry portable anti-tank weapons to handle other power armored threats.
Secondary weapon systems are usually built into the power armor's themselves in the form of machinepistols, lasers, missiles, and other weapons that can be fired quickly without getting in the way of what power armors are doing at the time.
Range, Accuracy, and Firepower have less to do with it than the purpose of the weapon being there. Try not to make just another gun and instead go for something that will be used.