Science knowledge was potentially lost in any post apocalypse settings. It does not mean that the scientific method was lost. Althought, it might have as in A Canticle for Leibowitz by Walter M. Miller which strangely enough does focus on knowledge preservation amongst other things. But I digress.
Mostly, you will want the players to do a little of evidence based practices and using some of their hard found logic to come up with ideas as to how to determine what the exact "bonus" the item provides. In your case of the +2 gun and assuming a 0-9 skill scale, it would be 20% more accurate in trials compared to a standard gun.
This works for some things but not others. What about that vial of glowing green liquid? Bio hazard or medical nanotech? How do you find out? By having a lab and doing some (careful?) experimentation. This could lead to a whole adventure: find the items, look for a lab and scientists, help them fix a problem they cannot solve but your party can (bandits?), then allow them access so they can experiment.
Do not look at special items as a +X. That's tedious. Look at how those special items do enhance your story. One of the James Bond tropes is that he will use all the gadgets he gets given by Q in the course of the film. They are there to facilitate the story, not be cool items on Bond's character sheet.