When using formulas like this, you should really tell the player what the damage is going to be. The dice roll expressions (NdM), although, imho, unnatural for computer games, are still pretty good at conveying this information to the players.

Not sure is it a problem for you or not, but if I had such complex airthmetic expressions for damage, I would do the following: for each weapon, show the histogram (the distribution) of the damage, something like this:

` # #`

# # # # #

# # # # # # # # #

1 2 3 4 5 6 7 8 9

Then, no matter how complex the computed expression is, the player can visually see the odds.

And the histogram can be sampled once from ~100-1000 trials, you don't really need to derive the formula for it. (Unless you do some wierd distributions like the one suggested by Hi, but you can really stick to the good-behaving ones)

Hi: Do you mean the Cauchy distribution?