The really easy way (and totally incorrect way) to read it is:
Bullets and laser zaps and plasma splats=0-200%
Explosions and melee=50-150%
One has a huge modifier, one does reliable damage. Many weapons break this mould though, even ones you'd think wouldn't. So check your anal(ysis).
Armour gets subtracted from the amount rolled.
But remember, Resistance comes in *before* armour. Have a 120 damage mortar, hitting a 75% concussive damage resistant target, with 80 bottom armour?
The very (very) maximum damage it could do would be "120" damage=(180x.75)-80. So, 55 MAX damage, but with a huge spread between "doing damage, and not doing anything".
Since I tend to spruke Assault Cannons, and even using solid cannonballs against some armoured targets early-on, let's see the horrible maths for that. "80" damage=Max=(120x.75)-80. So 10. Ten f'ing damage against a tank. At the very best damage rolls.
Note, none of these things are entirely correct. It just points out how strong resistance is, as well as high armour values. Piercing is better, because at higher values it strips armour. But not if other stuff can do it better, or can do damage better, against that target, etc, etc. And why big lasers (that ignore 33%'ish armour, sometimes, depending on weapon) are a fair bit stronger than they look. And why a gal with the Gold Codex with an Officer's Outfit and an XG Assault is in a special place (25% TU, 100'ish piercing damage snaps). Basically, X-Piratez is weird. It has heaps of caveats and awesome stuff. Stat blah isn't one of them.
A mix of weapons will serve you well. Big damage is always nice though.
Big armour is amazing too. But resistance plays a massive part in what does/doesn't work. Always hit them where and how it hurts!