Bullet Arts deal 0.5% each, and can be used after the initial animation of almost every attack (barring floor attacks, edge attack, throws, Bullet Climax, downward After Burner Kick, and Witch Time). The rapid firing can be held for a few seconds, and the bullets have moderate range.
The bullets themselves are actually invisible, since their hitboxes are similar to a grab aerial's instead of a typical projectile's. The bullets cause flinching at point-blank range, but otherwise deal no hitstun, much like Fox's Blaster, so they can be used to essentially "steal" KOs in timed matches.
Technically, the bullets are not projectiles, but rather indirect attacks with large disjointed hitboxes. As such, they cannot be reflected or blocked by a passive shield, though they can be slowed down by another Bayonetta using Witch Time. In Ultimate, due to Witch Time's duration now also depending on the strength of the triggering move, Witch Time won't last long enough to punish the other Bayonetta. Bullet Arts are also capable of traveling through multiple characters at once, and thus will hit anyone or anything in their line of fire. Even if blocked by someone's shield, the bullets will damage any opponents behind the shielding character.