Install 20 weapon mods.

Mods can be acquired from many traders in the waste, and then can be added to weapons via your Pip-Boy. Each gun can hold up to three mods. You can do this over time, but MUST be done in one playthrough. You can complete this immediately after completing The Courier Who Broke the Bank as then you'll be swimming in money. The Gun Runners have a good supply of mods to buy if you're searching.