Vault Boy icon in Fallout: New Vegas
Fallout Tactics Fast Shot Vault Boy icon
You don't have time to aim for a targeted attack, because you attack faster than normal people. It costs you one less action point for guns and thrown weapons.”— In-game description
Fallout, Fallout 2 and Fallout TacticsEdit
This trait reduces the Action Point cost of all throwing and firearm attacks by 1, but doesn't allow you to make Aimed Shots. Despite the description, in Fallout, it actually reduces the cost of all weapons, not just throwing and firearms, but this was fixed in the later games.
Fallout: New VegasEdit
This trait reduces accuracy with Energy Weapons and Guns by 20% while increasing the fire rate by 20%. This reduction in accuracy reduces the chance to hit when firing in V.A.T.S. and increases the amount of sway when taking zoomed shots out of V.A.T.S.
Fast Shot is arguably better for sniper weapons. For example, the anti-materiel rifle has a spread of 0.015 and a fire rate of 0.4. With Fast Shot, this would increase to 0.018, and the fire rate increases to 0.48, a negligible difference either way that would probably not see influential use. Conversely, automatic weapons, with their high base spread, suffer the most from this trait, and would only be required for more maintenance from all the extra wasted shots.
This trait works extremely well with other AP reduction perks like Math Wrath. For some weapons, the AP cost of weapons is reduced significantly to the point of allowing many more shots in V.A.T.S. An example of this is with the All-American, which has an AP cost of 13. When using the All-American with Fast Shot and the perk Math Wrath, the AP cost becomes 9.36, which can allow some characters 10+ shots in V.A.T.S. The extra shots in V.A.T.S. also makes the perk Concentrated Fire more appealing.
It is not advisable to choose Fast Shot along with Trigger Discipline. Fast Shot multiplies attack speed by 1.2 and accuracy by 0.8, while Trigger Discipline multiplies attack speed by 0.8 and accuracy by 1.2. So you end up with a 4% (= 100% - (100% * 0.8 * 1.2)) decrease of both accuracy and attack speed if both traits are taken.
pc Fast Shot does not appear to affect the rate of fire of automatic weapons when used in real time, only in VATS. [verified]
You don't have time to take aim because you're too busy firing off shots as fast as possible. It costs you one less Action Point to attack with firearms and thrown weapons, but you cannot make targeted attacks with firearms and thrown weapons.”— In-game description of the trait in Van Buren
J.E. Sawyer's Fallout RPGEdit
During the charge phase, the players character can fire a firearm instead of just charging ( This eliminates an action phase of that round), but critical hits with firearms always come in 6 point increments.