Fate Core offers no by-the-book default values. You're free to work out the value on your own (or burdened with doing so). You can look to specific Fate implementations for guidance.
You can simply treat them as having the same value to a +2. Strictly speaking they aren't — weapon:2 won't help you hit any more reliably — but it could be perfectly good for your group, and was for ours for a while. I picked weapon:2 over +2 for a half-troll, because I wanted him hitting harder, but not more reliable at hitting anyone.
Atomic Robo RPG provides a solid weight for Weapon and Armor ratings though via its hardware stunts (on page 75):
- Armor:2 is worth one stunt. This increases your ability to reduce the impact of attacks across the board, where it makes sense for your kind of armor.
- Weapon:2 is worth half a stunt. Add another half a stunt onto this. Your half a stunt can be something like:
- +1 to (action) with (skill) in (context), i.e. exactly like the +2 to a skill template but just +1 instead.
- Another weapon:2, for a total of weapon:4 in one stunt.
So weapon/armor hardware stunts might look like:
Stun gloves. Weapon:2, and I have +1 to Create Advantage when attempting to daze someone with Fight.
Father's hunting knife. Weapon:4.
Labtech™ Combat Armor. Armor:2.
Me trusty bashin' shield. Armor:1, Weapon:2.
You'll notice that unlike +2's, weapon/armor ratings don't require an action/context scope — that's because the hardware itself provides the scope. The stunt name always describes the hardware. Weapon ratings apply to any attack made using that hardware, and armor ratings apply whenever your armor is relevant in defending from an attack. (A flak jacket will stop a punch, but won't help against Provoke-based attacks.) Its take on these ratings is on page 124, but it's pretty short.
Weapon/armor ratings tend to be more broadly applicable than +2's, but you can only use them when your equipment's relevant, and they can't affect the roll itself (and help you succeed with style), just the shifts that come from it.
Atomic Robo probably makes weapon:2 cheaper than armor:2 for two reasons:
- To help beat the zero sum problem.
- Because armor:2 can wind up being used a lot more than weapon:2. You can only apply weapon:2 when you actually get to take an action to attack, but armor:2 can wind up applying to every attack you receive in a round! (And if you're the one with armor, you might be the one trying to bait the attacks.)