I'm using Wolph42's framework for RT, and on the settings there's an option to turn that rule on or off. In that option's description it states that it's rules as written for all FFG40k systems except for Dark Heresy.
It states that, when a character performs a test to make an attack, if the lowest damage die roll is smaller than the number of Degrees of Success, then the DoS value is applied instead.
For example, if Titus has a BS of 60 and rolls a 10, which means he has 5 DoS, but then his damage rolls (2d10) result in a 7 and a 1, his lowest die (1) will be replaced by the DoS (5), for a total damage of 7+5=12.
I've been looking for that rule in the RT core rulebook to no avail, and I can't recall hearing or reading about it before. Is this rule correct?
PS: I assume that if it's a single die weapon, the result would also be replaced by the DoS if they're higher than the damage roll.