Open Winchesterp94.esp in fo4edit go to the weapon tap and click on it.
The weapon tap will show two weapon below it one for minigun the other for the Winchester P94.
Click on Wincester P94 and you will see all the weapon stat on the right screen look for KWDA - Keywords slot you will see a keyword like this
[xx24a0d7] <Error: Could not be resolved>*
another one in the APPR - Attach Parent Slots
[xx24a0d8] <Error: Could not be resolved>*
*Remove both of this will get the weapon to be able to be moded in the weapon bench. It works for me.