Engraving/performance tuning is easily compared to "levelling up" a weapon. While the improvement can be very subtle sometimes but say you bought two identical rifles and engrave them the same performance path, after a lot of use ( I'm assuming online, haven't tried offline ) the performance bar fills up till it's maximized. Take a look at their stats then they are no longer identical. The stats acquired are sorta random ( niji explained this earlier in the ac5 discussion sometime ago ) so you would improve the rifle by random points. So far I've bought and fully tuned a laser cannon for accuracy ( not extreme accuracy ) and from the looks of it, my cannon always hits the target as long as it's in the lock ring compared to the first time I bought it which would sometimes fire slightly off target. I've yet to fully test it so don't hammer me if it comes out wrong.