Consider the following role with a Double barrel Shotgun:

Basically the damage rolled without any effects applied would be 6 (1+1+1+1+2) damage.

Now we apply the vicious effect which gives us 1 extra damage for each effect rolled, so for 3 effects that makes 3 extra damage which brings the total to 6+3=9 damage!

Next we apply the Spread affect which gives us an additional hit for each effect rolled with half the rolled damage (rounded down).
Here is my question, does this mean I take 9/2=4 or 6/2=3 damage?

Assuming it is 9 that one hit would do (without damage reduction) 9 + 4 + 4 + 4 = 21 damage?

Did I miss anything or is the combination of Spread and Vicious really good?

Spread: For each Effect rolled, your attack inflicts one additional hit on the target. Each additional hit inflicts half the rolled damage (round down) and hits a random location even if a specific location was targeted for the initial attack.

Vicious: The attack inflicts +1 damage for each Effect rolled.

I think itâ€™s 9 + 3 + 3 + 3 = 18 (â€śrolledâ€ť damage = 6 for spread). There is a reddit post about this, and yes, the consensus seems to be that shotguns are quite powerfulâ€¦ if the person being shot doesnâ€™t have any mitigation: https://www.reddit.com/r/Fallout2d20/comments/nhz4mv/rulesaswritten_effects_stacking/ The post and comments discuss that, and even give an example where a good shotgun roll gives 11 damage, but mitigation reduces the spread hits to 0.

Fairly certain that Vicious counts as rolled damage. There have been clarification here and on discord that Vicious is an exception to the rule that effects do not transfer to additional hits.

Also note the example roll is fairly high.
The average on 5 dice with Vicious is 5.83 damage and 1 2/3 effects.
Round that to 6 damage and 2 effects and total becomes 6+3+3 = 12

Now where Double Barreled Shotgun becomes frightening is at high levels. Unlike the combat shotgun the double can have an advanced receiver AND vicious.

Maxed out you get 8CD, RoF 2, range M not inaccurate and reflex sight allows reroll hit location. Add Shotgun Surgeon and Rifleman 2 perk and you have +2CD and Penetrating 2. That gives an average (using full RoF) of 12CD for 14 damage and 4 effects. Total of 14 (pen 8) + 7 + 7 +7 + 7 = 42.

I would say as a house rule that the DB Shotgun should use 2 ammo per shot by default, or lose vicious if only one ammo used.