I suggest you download this xEdit script, as it makes adding a lot of modded weapons at once easier:
http://www.nexusmods.com/fallout4/mods/13565/?
You can also take a look at the Fallout: New Vegas wiki to get an idea of how much strength your modded weapon should have.
Step 1: Load my mod and any weapon mods/required masters 
Step 2: Deep copy my mods FormID records into a NEW .esp
Step 2a:
Step 3: CTRL+CLICK the records you want to copy into the form lists
Step 3a:
Step 3b:
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I rarely have to do this, but certain weapons like DOOMs Desert Eagle, or the varmint rifle have custom aim models. If your weapon mod has a Aim Model category than copy the records as an overwrite into a new file and change the aim model record entry "Cone of Fire - Iron Sights Mult" value to 0.8~1.0. I suggest checking all your weapon mods and overwriting them into a SINGLE file so you're not loading 100 extra esp.