Image: Naughty Dog / PlayStationOfficial screenshot from Naughty Dog / PlayStation.
Combat Ready is the silver trophy for maxing out every upgrade on one firearm at a workbench. You need enough parts, and for the later tiers a high enough tool level from the toolboxes you have found.
Pick one gun and pour every part into it. Missing early toolboxes caps your tool level, which can block the last upgrades on a weapon until you find more.