Kinda unsure how combat works, and I wanted to see if I had everything correct.

From the standpoint of a devastor marine, on attack with a standard Heavy Bolter

With an BS of 50

I’m assuming here that a turn was taken to brace the weapon, although, with bulging biceps, I’m not sure it’s necessary.

1. Attack roll is 12
2. BS plus the bonus for full auto is 70
3. Giving the devastor marine 6 total successes
4. Target rolls a dodge AGI 35
5. Rolls a 10
6. Target successfully dodges 2 “hits”
7. Damage is rolled for 4 “hits”
8. Now things get weird, according the core rulebook no damage “die” can be less than the total number of successes, so by default, any roll of less than 6 is 6 damage.
9. So assuming with a heavy bolter 2d10 damage per hit minimum he can do (before bonus’s) is 12 damage. And if any of those 8 rolled dice get a 10, he has the possibility of triggering righteous fury. This to me seems like an insane amount of damage by default

Am I missing something here, or is there something I’m doing wrong?

You've got it right according to the rules. I think the minimum damage rule may have been clarified to work only for the first attack, but I don't have proof of this on hand at the moment. Unless I can find that clarification I say don't worry about it and go ahead an play your damage according to your example.

You were right up to step 8. Also, yes, with Bulging Biceps, there is no need for the Brace action.

The mistake made on step 8 is that DoS thing is tied to only one die of damage. The intent is to allow a sniper type firing only a single shot to not derp the damage roll on what should otherwise be a powerful hit. Its there to allow that attack roll of 01 to still be damaging despite the player rolling a 1 on damage.

I can see in DW how the wording is a bit screwey, "The number of Degrees of Success is the minimum amount of Damage that attack will inflict on a single dice." I think the problem is a spelling error, they should have written "die."

Only War does a better job of clarifiying this rule:

"The attacker may choose to replace the result of a single Damage die with the number of Degrees of Success from his attack roll."

Either way, I would only allow the DoS replacement on one die, and not have it termed as "minimum damage," but use the concept of replacing the rolled value. Its worth noting here that a Righteous Fury still requires a natural 10.

The line following what you posted from the DW rulebook Komissar specifically states when rolling multiple dice for damage only 1 of the dice can be replaced with the "minimum damage".

Now that you've mentioned it though, I remember what was clarified earlier. Only 1 dice of all the damage dice rolled with a single attack may be replaced with the "minimum damage" result.

This means the OP's example of hitting 4 times with a DoS 6 attack, only a single dice from all 4 of his hits may be replaced with a 6. So if you rolled 2 1s a 7 and a 9 for your 4 damage rolls, then you can replace either of the 1s with a 6, but not both.

small correction, I think you have 5 degrees of success

Korwus said:

While true, that doesn't take into account that the enemy succeeded on a dodge as well.
Actually looking at it further, nobody caught that the enemy dodge should avoid more shots than it does.

To go back over all of it:
BS Target = 70, rolled 12. As this is a "success + DoS" system (as opposed to BC/OW's "DoS only" system), then it is:
12 = success, 22, 32, 42, 52, 62, 5 DoSs.

6 shots total will hit from a heavy bolter (too lazy to pull up the errata weapon stats but this should certainly be possible with the CRB listing for the HB).

Dodge, target 35, rolled a 10.
10 = success, 20, 30 = 2 DoSs, so 3 hits dodged.

So it should be a total of 3 hits.