Hitscans are fired by all bullet-firing weapons in the Call of Duty series, apart from the PTRS-41 anti-material rifle in Call of Duty, the Barrett .50cal in "One Shot One Kill" in Call of Duty 4: Modern Warfare, the Barrett .50 Cal with a Thermal Scope in "Of Their Own Accord" in Call of Duty: Modern Warfare 2, the RSASS in "Blood Brothers" in Call of Duty: Modern Warfare 3, certain pistols in Call of Duty: Black Ops II, and some machine gun turrets. They travel in perfectly straight lines at infinite velocities and are infinitely thin. They disappear once all damage is lost due to wall penetration or when they hit a level's boundary. In the interest of balance, shotgun hitscans disappear after passing a certain distance, making it impossible to hit a target that is beyond a certain distance from the shooter.
Because the hitscans travel at infinite velocity and in perfectly straight lines, bullet drop and the target's movement does not need to be compensated for. This means that aiming above/in front of a target does not need to be done to hit a moving target or a target at long range (except in certain cases where lag may also be a factor).
In Call of Duty: Modern Warfare 2, every few hitscans fired will be a tracer round, it will show up as a bright yellow flash zig-zagging towards whatever is being shot at. This effect is much more noticeable with a Silencer, which eliminates muzzle flash, and FMJ, which actually increases the frequency at which these tracer rounds are generated. Note that tracer rounds in this sense refers to a purely aesthetic effect, and not an actual in-game entity.
In Call of Duty: Black Ops, all hitscans are possible to see unlike the previous Call of Duty games (except for shotguns and certain weapons involved in previous games). This is easily seen comparing the sniper rifle shots from Black Ops to sniper rifles shots from the previous games.
Call of Duty: Black Ops II introduces bullet trails exclusively for the sniper rifles. When a sniper rifle round is shot, all other players will see a distinct smokey trail coming from where the hitscan was shot, all the way to its final destination. The trail will slowly rise up and disappear a while after it appears.