So, let’s skip the question, most variats of AR family can be “created” by your own. So yes, those weapons (primarily the AR series) are fully and maybe over modifiable.

I am forced to be familiar with parent linking system of FO4, even when my first time creating a weapon, there are only few problems appeared. WHAT is AR? I mean, I know what AR is, but WHICH AR exactly is in your mod? But whether you are an AR love or hater, you can’t just ignore that they are “commonly seen” in North America, so that is the first thing I made in this project. I am not living in North-America, yet according to my knowledge, the most popular and wildly used firearm over military and civilian market in the pre-war U.S., probably to be the AR family. The “Great War” was happened in the last quarter of 21 century, but judging by the pre-war weapons that existed, the 21 century itself seemed never existed.

I am not an expert of background settings, but I do think we always received old-fashioned and improper weapon in the Fallout universe. This mod new modern weapon AR, ACR, H&K MP5, HK416/417, H&K G36, FN SCAR-L/H, SIG SAUER SG550 Series, AK for Fallout 4.

Modern firearms fallout 4 reddit