Allright, let me explain this properly.
The balance of weapons presumes that weapons of same level have same or nearly same "weight". The "weight" is determined as a summ of weapon's advantages and disadvantages (if any). Thus we have to 1) Define target weights for each weapon level - that is a weight all weapons of that level should have, and 2) Define weight values per each weapon property. A summ of weapon properties will give weapon weight. It is up to us to make every weapon fit the level.
Just for example, we have decided that level 4 weapon should have roughly 40 weight units.
We are taking Fighter's Ripper for balancing.
At first, we calculate the base of its weight using those factor tables (remember, everything is for example only):
Class is : "fast projectile" = 9
Fire rate is "slow" (24 tics) = 2
Single main missile = 1
Area of attack: straight = 1
total = 14
8 extra missiles spawned on hit:
Class is "slow projectile" = 3
Area of attack: spread = 2
total = 5 * 8 = 40
Grand Total = 14 + 40 = 54.
So we have weapon weight 54 and limit 40. Now we still have to set damage so that its weight won't exceed limit (much), but it is already exceeded. So we have to use disadvantages, that is apply large mana cost (I forgot to mention it in the tables); or maybe decrease attack speed/projectile speed to lower down basic weight.
That is the idea.
The only thing I don't understand is, what do you mean with 'damage level' and what do 'A to B', 'C to D' etc. mean?
Its just a template. We may determine roughly standart damage levels, like 10 to 15 of damage, 15 to 25 etc.