The new automatic rifle actually has great damage, but .308 ammo is too expensive and rare. The 10mm SMG is another good automatic gun, it has effective power and good accuracy. In addition, 10mm is cheap and easy to find. The best stealth machine gun is the 12.7mm SMG. Off the top of my head, it is the only one that can be silenced and has great damage for very quick close quarters kills. The final automatic gun I would suggest is the LMG. It has good accuracy, damage, and ammo capacity. It's incredibly easy to hunker down and blast enemies continuously at any range. That being said, you are going to want to watch your ammo supplies carefully. Depends on what you're doing, but I personally feel pretty good about the BAR with armor piercing rounds. Drop a Deathclaw fast!
If we're speaking purely about DPS the minigun with AP rounds beats them all but it has the highest strength requirement in the game, weighs a lot and eats ammo by heaps.
^^ Yeah I agree the B.A.R. is great on deathclaws, but I'd go with the light machine gun. I killed about 10 Legion Assassins and Veterans combined with 10 rounds left. No expanded drums.
The .45 auto SMG with all the perks (bloody mess, grunt) with AP rounds tears through shit like a monster. even better with the round drum and businesswear, hat, and sunglasses. Mafia style.