Overview
Basically a powercrept Defender. The DMG debuff and HP Shields both want her to have a Choke equipped at all times, as her base ammo is only 5. With a Choke she will gain 80 HP Shields per reload and will inflict her little debuffs.
As opposed to Defender, 520 can actually make use of both Birdshot and Slugs, allowing her to hit up to 5 targets or focus on a single one, depending on the need. Of course, she will likely be using Flash ammo for most things.
The debuff only lasting 1s seems a bit weird and limits how useful she is, but can be considered a nice bonus. It does not require hitting the target.
Do not leave 520 next to other Dolls (not counting diagonals) at the start of battle, as she'll lose her reload reduction. If that happens, a Choke will further increase her reload time because it is based on her actual ammo count rather than her base ammo, unlike other SGs. This is likely a bug but doesn't really matter since it only happens if you use her incorrectly.