Jax, DaVeg, I know the old gun part system inside and out. It was very limiting.
All you are hearing seems to be "removal of gun parts". Well think of it this way, each of those parts were basically a limited version of what mods are. The new mod system even uses the old assembly screen.
Only one actual item ID in the xml has to be used for all item mods. The mods are stored in another xml file and have an upper limit of 32,000(ish) possible mods. In the old gun system a pistol could only have a single item assigned as it's "barrel"
Now, using tagging and mods, a pistol COULD have 50 different types of barrels. All that do various things aside from editing the same set of stats. One could add damage and take away magazine size; One could have less damage but give you 1hp per shot when you hit an entity; One could give you any level in any skill for a specified time after a kill. 3 off the top of my head that are possible in the new mod system, and that's just scratching the surface..
The limited nature of gun "parts" evolved into a mod system that can now work on any item in the game, not just tools and weapons. The new system can mimic the old system exactly if that is the desire, but, it can also do much much more.