In Titanfall 2, Weapon attachments are considered to be anything that changes the sights of your weapon.
Irondot Sights (sometimes referred to as Iron-Dot Sights) are specialised sight technology employed on many weapons found on the Frontier, as part of their factory-issue loadout. They augment traditional Iron Sights with holographic displays using "Floating Pixel" technology, showing information such as reticules and ammunition readouts. They are the default optic fitted on all non-sniper and non-grenadier weapons in Titanfall 2.
The Holosight returns in Titanfall 2, though not as widespread as in the first game. The sight is now only available for SMGs and Shotguns (with the exception of the Spitfire LMG found in the campaign mission The Ark). It now features a 2.1x magnifier scope to help magnification, rather than the concentric rings of the first game.
The Threat Scope is the name for two different kinds of attachment in Titanfall 2. They both take the form of optics providing an AR overlay on their scopes, highlighting enemy troops in red.
However, the main difference is in size; assault rifles, SMGs, Light Machine Guns and shotguns all get one that is more comparable to a Holographic Sight, while Sniper and Marksman weapons get a full scope. Every single primary weapon, with the exception of the Grenadier class, has compatibility with one variant of threat scope or the other.
Due to the power offered by highlighting enemies, Threat Scopes also take up the second mod slot for their weapon, meaning a Pilot must choose between having two mods and another sight, or a single mod and the Threat Scope.
The Holographic Combat Optical Gunsight (HCOG) is a weapon attachment that returns in Titanfall 2, although with a massive visual upgrade. The sight housing is now much wider than before, and is only available for use on assault rifles and LMGs. Like the AOG, the HCOG now has a holographic line that slowly trickles down when bullets are fired until the weapon needs reloading.
The actual holographic reticule is the same as that of Titanfall, except red.
The Holographic Combat Optical Gunsight Ranger, also known as the HCOG Ranger or more formally, the Advanced Combat GunSight (ACGS), is an optic attachment available for Pilots in Titanfall 2. It functions as a more advanced version of the standard HCOG, able to zoom in to a much higher level.
The scope is functionally the same as in Titanfall. However, its functionality has largely been replaced by the HCOG Ranger, leading to it only being available for Light Machine Guns. The R-101C Carbine, added in Colony Reborn, has an AOG scope as the default Factory Issue sights, but the default zoom level of iron sights.
Talon Variable Zoom Scope
The Talon Variable Zoom Scope is a sniper scope available for use on the Kraber-AP Sniper, Longbow-DMR Sniper and D-2 Double Take. It allows the user to change the zoom level of their sniper scope, allowing the weapon to be versatile at both medium and long ranges.