I miss finding and upgrading gun parts.

The gun part system did not work in conjunction with the weapon mod system as they both utilized the same interface. It basically came down to being one or the other. Since they were getting rid of the upgrade system with the 600-point curve due to abuse on the workbench already, it made sense to get rid of the parts as well. This also helped to trim down the number of items in the game as there were code limitations at the time that were preventing the introduction of new items as the index cap was hit.