Damage Types are a good way to add variety to weapons and armour in USR. Armour protects against a specific type of damage, and for every level of armour you have, the damage level of the attack drops by one.
Generally, I like to limit damage types to four or less. Here's what I usually use:
Melee, Shrapnel, Fire, Energy
The first two are pretty straight forwards, one damage type is for melee and one for general ranged weapons. Fire is for explosives and other firey attacks and Energy is for stuff like lasers and plasma. Depending on the setting you're going with, you might want to have more damage types that fit melee weapons. You could also swap them out for elements too! I recommend sticking to 3 elements or so, having a lot can be hard to remember and it makes it a gamble to use a weapon of a rarer type since it won't be obvious how often it'll be useful.
These also work really well for weaknesses. You can throw tougher enemies at the players and have them figure out what it's weak against.
The USR (Unbelievably Simple Roleplaying) System belongs to Scott Malthouse and Trollish Delver Games