If Biggy's Reputable Weaponry doesn't have the upgrade you want, you will need to find and learn the blueprint yourself. You will want to scour Desert and Forest planets for the Boomerangs, Chakrams and Gauntlets, though Risky and Dangerous difficulty planets can also have these items. Though legendary weapons found on Extreme and Inconceivable danger planets are very good, don't forget about the craftable upgrades to Boomerangs, Chakrams and Gauntlets (all of them are equivalent to tier 5 legendary weapons). If you have a bunch of Snowflakes from going through Ice biomes, Snowflake Shurikens, crafted at an Anvil/Forge/Replicator, trade some damage and fire rate for the ability to chill enemies.
Those things can be spammed like nothing else, especially if you have them in both hands. If projectile spam fits your style better, you should farm tier 5 planets for Throwing Stars. The downside is that you can only throw them from a given hand once per 2 seconds (throwing with both hands lets you get a net fire rate of 1 per second). Those things do 100 damage each before any damage bonuses are factored in.
