I think I figured out the barrage rules somewhat. It's actually in the text, but you have to really work to get this from what's printed.
Under the paragraph headed by "Barrages and Defences" there is a sentence that gives a clue how the system is supposed to work. I quote,
High Guard said:
Add up the protection offered by the defences, and then subtract it from the individual weapon damage score to determine the final DM.
Ok, so there could be a large number of ways to do this. I settled on something that seems to work for me and is simple. The first step is to determine the initial DM with armor only, and then account for sand, point defense, and the like as the book discusses.
Here's what you do: Subtract Weapon damage from Armor and keep the sign (positive or negative). Then work from there. Why does this help? Consider how barrages work--the higher the total roll, the more damage you do. So, if you have a weapon that does lots of damage vs small armor, it ought to be easy to score lots of damage. If the armor outclasses the weapon, it ought to be the other way around. Currently, it's just not clear that this happens because all these DMs get lumped in together and you don't seem to get any bonus for having a weapon that outclasses armor. This interpretation changes that.
So, to calculate the Protection DM, start by subtracting WEAPON DAMAGE from ARMOR rating.
Examples:
Beam laser barrage vs. 10 Armor --> 10-1 = 9. Initial Protection DM from Armor alone is +9, then you add in sand clouds. A beam laser is not likely to do much against this much armor.
Particle Beam Barbette barrage vs. 10 armor --> 10-4 = 6. See how this begins to work?
Now finally, it works even better with high damage vs. lower armor.
Particle Beam Heavy Bay vs. Armor 4 --> 4-9 = -5. Apply the rest of the defense DMs as appropriate, which are positive numbers. If you don't have enough other defense DMs, your armor is actually giving a BONUS to the attacker. This means that heavy weapons will SMASH through lightly armored craft using Barrage attacks, exactly as it should be.
What do you guys think?