Just an excerpt from the AliensRL manual:
== Armor ==
Armor is your only protection against an alien's sharp claws, so you'll
want to make sure to wear one at all times. The most straight forward
gain from wearing armor is the damage reduction, which differs between
different kinds of armor.
The number behind the armor represents this damage reduction, e.g. "Armor
: Flak armor [2]" means, that the flak armor has a damage reduction of 2,
which is more than a simple flack jacket has. In addition to protection,
armor can offer you better offensive capabilities by allowing you to equip,
and use, heavy weapons. The weakest armor with such a bonus is the
gyro-stabilizer. It offers no damage reduction but instead grants you the
ability to equip and fire a heavy weapon. The choice between taking less
damage and dealing more is tough, and it's all yours. Choose wisely, Marine!
Perhaps the manual should just say "Gyro"? I can understand there being a bit of confusion, but it is written in there, at least.
As to AliensRL - you can't approach it like DoomRL and expect to have the same experience. AliensRL and DoomRL are not in quite the same genre. DoomRL is about blowing the hell out demons of hell (making them demons of nothing?), AliensRL is about trying to stay alive long enough to kill the Queen and get out. DoomRL is more action, AliensRL is more survival/horror. DoomRL it's expected that since you're fighting demons, you'll die once in awhile. AliensRL you're fighting Aliens. They're smart, they're coordinated, and they've got a base of operations going. You're screwed from the start.
It can be a lot of fun if you're willing to let it, but AliensRL is NOT a 'coffee break' roguelike in my mind.