Earlier this month, Fallout 3 mod maker Zealotlee — known for the ACR and contributions to Project Nevada for New Vegas — revealed that he has already started working on a mod for Fallout 4. It'll come in the form of the Rail Rifle, which we got a preview of earlier this month.
Zealotlee returned to Bethesda's forums today to provide a progress update. "The majority of the low poly models/normal maps are complete with select pieces remaining," Zealotlee wrote, adding that texturing "should begin soon" and that it will be "a healthy mix of metal and wood where applicable."
According to previous details from Zealotlee, the Rail Rifle will have multiple modifications available, with each mod altering the guns' performance. So far, you'll be able to choose different barrels, muzzle breaks, stocks, and grips, as well as optics. Each barrel and modifications will have "some sort of effect." For example, the long marksman barrel will have a lower rate of fire, but higher accuracy and damage.
The Rail Rifle will use energy weapon ammo. EC Cells, assuming they are still in the game, will be used for the light ammo drum which has fully automatic fire but does less damage per shot and is less accurate. MF cells will be used for the heavy ammo, which is only semi-auto but does more damage per shot and is more accurate.