Faraday Slug
Increases headshot lethality.
The Energy equivalent to Hollow Point.
Fusion Mag
Enhances energy ammo recharge rate, even when stowed.
The Energy analog to Extended Mag. Instead of larger magazines, you get effectively infinite magazines. Energy weapons regenerate ammunition in the active mag by default, but Fusion Mag essentially doubles this rate.
This sounds great on paper, and it is quite useful, but keep in mind that the rate of regen is slow enough that it tends to favor slower firing and longer-range weaponry.
Close-range bullet hoses, like an energy SMG, tend to drain ammo too quickly for you to reliably expect the Fusion Mag to keep up with the demand.
However, it is better to have a regenerating SMG in your pocket while you fight with a pistol or stolen weapon than no ammo whatsoever. If you aren’t interested in running the Scavenger perk, Fusion Mag can provide a viable alternative.
In general, the Scavenger perk can provide you with more than enough ammo, but it favors more aggressive builds. If you want to keep your distance, Fusion Mag is more useful, because there’s no need to expose yourself to risk to pick up Scavenger packs. You can simply hang back and let your Fusion Mag top off your ammo between kills.
Supressor
Invisible on radar when firing and reduces muzzle flash. Shortens effective damage range.
The Energy equivalent to the Suppressor.
Particle Amp
Increases stopping power at long range.
The Energy equivalent to the Rifled Barrel.
Ram Servo
Creates additional reflections on energy shots. Increases damage against equipment.
The Ram Servo is functionally identical to FMJ when it comes to tackling enemy scorestreaks and equipment, but instead of a bonus to cover penetration, the Ram Servo instead gives your particle shots enhanced ability to ricochet off walls.
This can be effective in tight quarters, where bouncing bullets might score you a few free kills that you might not have secured otherwise.
Trying to intentionally make use of the Ram Servo to get kills is possible, but very ammunition intensive, so make sure you have the Scavenger perk, Rewind trait, or Fusion Mag attachment to deal with the ammo drain if you plan to actively go for ricocheting kills.