Performance Tool W5239 Polymer Viton O-Ring Assortment, Brown, 180 Pieces, 9mm-20mm https://www.amazon.com/dp/B01N8ZLHQO/

Not technically a tool, but this set has saved me time and money several times over the years.

A hollow-ground set of screwdrivers, also a gunsmithing screwdriver set. The blades are ground different than your normal tapered tip. They fit the flat head screws better and prevent camming out and scratching the shell. Also, get a proper set of metric pipe fitting wrenches so you do not twist the tubing.

I wonder if you might be talking about flare nut wrenches.