I want to tell you why I think Aspects are awesome, but first, I want to talk about armor penetrating ammunition.
For those of you who aren't hip to such things, here's how it works:
A standard bullet leaves the barrel of the gun moving really fraking fast. Now, lead is a heavy metal, but it is also soft. When it strikes the target, the bullet will "mushroom". It loses it's aerodynamic shape, becoming more of a shapeless mass that rips through the body, possibly even bouncing around a bit before tearing its way out. Assuming it doesn't get stuck in the body.
And there is a good chance that the bullet will break up into multiple pieces, spreading the damage throughout the body and making extraction of the round even more difficult. Note that "hollow point" rounds are designed to mushroom in the air before hitting the target, further ensuring the hapless victim is going to have a bad day.
On the other hand, bullets designed to penetrate armor (often called steel jacketed rounds, or "full metal jackets") are wrapped in a casing that not only hardens the bullet, but ensures it maintains it's aerodynamic shape, which just happens to also be the shape of a deadly spike.
Now, when a regular bullet strikes a hard surface, or an armored target, the armor causes the rather soft bullet to break apart. It disperses its energy and mitigates the chance of penetration. But the harder armor piercing round is resistant to breaking up, so much more of it's energy stays directed toward the task of pushing it's way through the armor.
This is really handy when shooting at hard targets, but not so handy when shooting at soft fleshy ones. When shooting at an unarmored person, the armor piercing round tends to punch through the body with minimal (if any) deformation. So the bullet tends to exit the body rather cleanly. I mean, you're still left with a seeping hole in your body, but at least it's rather neat and tidy.
So how do you model that in a game? Well, up until just recently we modeled it in Strands the way it is handled in many other games. Certain weapons had an Armor Penetration value, and that value was subtracted from the target's Armor Rating before stress was calculated. It's how most games do it, and changing the AP value is a good way to differentiate between weapons.
But I hated it. How many times have you heard "Oh! I just remembered I have an AP rating of 2, so go back and subtract 2 more hit points from that guy for every time I shot him", or something similar?
Ugh. What a pain.
And then it hit me! Armor Penetrating is just about the perfect Aspect to put on weapons. Whether it's with a steel jacketed round or not, a gun shot usually screws you up pretty bad; or just kills you. But if you are wearing armor, armor penetrating rounds are probably more effective. If not, it might not be as effective as a regular round would be.
Like so many other sub-systems that other games struggle with, in FATE, often it's so easily modeled with just an Aspect. So just spend the Fate Point and invoke it if you want it to matter. Invoke for bonus on attack rolls against armored opponents, or your opponent can compel it for a bonus on their defense if they're unarmored.