Those aren't the critical values, those are the criticals added in.
Ah, I see it. Now going by that format.
Brush Gun damage: 75
Brush Gun Critical damage: 150
Toss in the Cowboy Perk and HP rounds, or SWC hand loads, it becomes clear why there isn't a scope mod for it. I'm getting a DAM rating of 168 currently with the brush gun with HP ammo and 118 with SWC rounds. Playing around with it I shot Veronica, who is wearing T51B+Vault helmet for +28DT, with HP ammo in the chest and a single normal critical hit killed her while at max health on normal difficulty. That is at least 299 damage there (215+(28*3)), so a sneak attack to the head would yield 1,196 damage (299*2*2) with the same DT. Yeah, scope mod for the brush gun would make the AMR useless.
Point is the AMR and Sniper Rifle are not the hardest hitting guns in the game. If your are good with iron sights and are willing to abit get closer the Brush Gun is king, but it's massive overkill 99% of the time. For me the Trail Carbine with SWC rounds does the trick for everything that's not a deathclaw or sentry bot.