The step damage system was something like this:
For step 7 you would roll 2d6 as the average is 3.5 per die and that adds up to 7 (for non exploding or re-rolling dice). With exploding dice this is d12.
Then, each time you rolled the maximum number on a die you rolled that die again. The step chart was supposed to roll the number in the middle more often. It was more linear than a bell curve.
While this seems a poor explanation, I hope that it will help.
TLDR:I am seeking the system that has the step damage as it's damage system.