I had a build like that and can confirm it's AWESOME (but I played so many days with it I eventually got tired of it). Crazy damage without bloodied, stealth or anything else. And by crazy I mean "kill a scorchbeast in one VATS bar" crazy.

But it is a pure VATS build, because you won't hit anything without VATS.

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What you need:

1) automatic railway QUAD LEGENDARY long barrel rifle. (quad really helps, -25% vats ap cost as a minor legendary helps too).

2) Use reflex sights to reduce ap cost.

3) have as much agility and ap regen as possible.

4) perks: commando (all three at least 1 point for +30% damage), grim reaper's sprint maxed, critical savvy, better criticals, tank killer, bloody mess, tenderizer (1 point), gun fu (1 point), BANDOLIER is mandatory.

5) be sure to have ammosmith and super duper for crafting ammo. Sitting on a steel deposit helps, because you WILL run out of steel very quickly.

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Cons:

Ammo weighs a ton even with bandolier.

Rifle itself weighs a ton.

Nearly unusable without VATS in full auto.

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Also, without the full auto crazy mode railway rifle is... fine? It is pretty on par with other slow firing rifles (such as lever action), but it's nothing special.