I have recreated this thread, because it got merged with other threads by accident.
NOTE: If you have any research on any of these attachments that would add additional info, post it in the thread and I'll update this post.
Hey! Because attachments are so poorly explained in-game, I've made this very quick guide, so people know what each attachment does. I've bolded the ones that are especially confusing.
Quick Shot Sight - A regular sight with a slight zoom
Holographic Scope - Another regular sight, with an increased zoom.
ACOG - A medium-range sight with medium magnification.
Clear Sight - A regular sight without zoom, also displays your remaining ammo.
Detecting Sight - Doesn't display enemies through walls. Instead, all enemies are highlighted in red, even through smoke and debris, but only if there are no walls or other obstacles in the way.
Monitoring Sight - Displays remaining enemy health and also the range to the object you're pointing at.
Glow-On Sight - A weaker version of the Detecting Sight, lights up in red when you're aiming at the enemy.
Rifle Scope - A regular sight with the largest magnification on non-sniper rifles.
All Sniper Scopes EXCEPT the Thermal Scope - Various magnification levels and reticles.
Thermal Scope - Low magnification with a red-dot, also has a screen that displays the body area you've hit.
Laser Sight - Tightens hip-fire spread.
Silencer - You're not displayed on the enemy's mini-map while shooting. Has no drawback, despite the stats stating otherwise.
Precision Barrel - Increases your maximum range, you will deal more damage on medium-to-long distances.
Modified Rifling - Increases your maximum effective range, you will deal more damage on close-to-medium distances.
Muzzle Break - Reduces horizontal recoil. (side-to-side recoil) (Possibly has no effect, or is very unnoticeable)
Compensator - Reduces vertical recoil. (upwards recoil) (Possibly has no effect, or is very unnoticeable)
Extended Magazine - Increases your magazine capacity, while reducing your ammo carried slightly.
Hair Trigger - Increases your rate-of-fire on semi-automatic rifles. You can press the left mouse button faster.
Straight Pull - Decreases the time it takes to load another shot into a bolt-action rifle.
Soft-Point Bullet - Increased aim-punch. (thanks Gnomedalf) Doesn't increase damage nor range.
Armor-Piercing Bullet - Decreases the penalty for shooting through walls by 50%. Allows SMGs to shoot through walls. (thanks Gnomedalf) Doesn't increase damage to drones.