found this on Wikipedia... pictures of the Assault Rifle with a small note from the developers about it

is there more? is it maybe a book from a Collector's Edition or something?

http://vignette1.wikia.nocookie.net/fallout/images/4/48/Fo4_assault_rifle_concept_art.png/revision/latest?cb=20160104193503

concept art:

http://fallout.wikia.com/wiki/File:Fo4_machine_gun_concept_art.jpg

Well that confirms it, it was designed like that to be used with Power Armor. I wonder why they changed it from Machine Gun to Assault Rifle?

ah I found it

it seems to be from 'the Art of Fallout 4'

it's a shame, that they sell this book for so much

but it also makes you see once again how much work a game like this takes

http://www.hardcoegamer.com/wp-content/uploads/2015/10/10.jpg

look at the details it must have taken several weeks to create something like this ^^