Rate of Fire (or Fire Rate) is a weapon stat, and represents how fast a weapon will shoot when the trigger is pulled.
Numeric Rate of Fire
Every weapon has a numeric Rate of Fire. This is roughly the number of times the weapon can be fired in 1 second. For example, a Sniper Rifle with a 0.4 Fire Rate will fire once every 2.5 seconds. An SMG with a Fire Rate of 4 will fire 4 times per second.
Semi Automatic vs Fully Automatic
A Fully Automatic weapon will re-fire as soon as possible, so long as you keep your finger on your trigger. In this sense, it will make full use of its rate of fire.
A Semi-Automatic weapon will only fire a shot when you pull the trigger. This will give it an overall slightly slower Rate of Fire then its numeric capacity, as you will always fire a split second after the weapon has fired.
It is possible to fire as fast as a semi-automatic weapon will allow by firing, releasing the fire button and then pressing and holding it before the next shot can be fired. This will fire the next shot on the earliest possible frame. This pattern can be repeated until the magazine is empty.
Weapons with burst fire have a very misleading fire rate. These weapons fire several bullets in quick succession with each pull of the trigger, rather than fire continuously while the trigger is held down, as a Machine Gun would. The given fire rate describes how quickly shots will be fired during a single burst, but it provides no indication of how long the pause between bursts will last.
Assault Rifles are a typical example of this burst fire discrepancy. They will fire in short bursts of (usually) 3 bullets, and then stop for an amount of time. Even though an Assault Rifle may have a fire rate of 10, if it can only fire a burst every 1 second, its true fire rate will actually be 3.3. Unfortunately, there is no way to know this 'burst fire rate' but to equip the weapon and try it.
This is true of any weapon with "increase burst fire count", such as some machine pistols. The rate of fire will apply only during bursts, and gives no indication of the actual time between two bursts.
Relation to Frame Rate
Fire rate stats and boosters may suggest a stepless scale, but in reality there is a limited number of possible fire rates. The engine of Borderlands games fires one shot per N frames, where N must be a whole number: one shot every 11 frames, every 10 frames, every 9 frames and so on.
If the nominal fire rate results in a fraction of a frame, e.g. one shot every 6.4 frames, it will be rounded up to the next higher whole number, in this case 7 frames – decreasing fire rate because more frames per shot = lower fire rate. In other words, nominal fire rate is always rounded down to the next lower possible actual fire rate.
This has several effects on gameplay:
- Actual fire rate depends on frame rate. A nominal fire rate of 8.7/s will be reduced to an actual 7.5/s at 30fps, to 8.6/s at 60fps, and to 8.0/s at 72fps. This also means two weapons may have different actual fire rates at one frame rate but the same at another.
- The higher the frame rate, the smaller the steps between possible fire rates, and thus the smaller the amount reduced on average. Hence, higher frame rate results in higher average fire rate.
- At low fire rates, the steps are small and difficult to notice, but at high ones, they become bigger and more apparent. There is little difference between one shot every 16 vs. 17 frames, but a great one between every 3 vs. 4.
- The lower the frame rate, the more pronounced this effect. At 30fps, even a nominal 9.9/s will be reduced to an actual 7.5/s. At 60fps, it will be reduced to 8.6/s.
- Fire rate boosts from skills, badass ranks, class mods etc. may yield unexpected results. For example, at 60fps, a +2% boost will give a 9.9/s weapon a considerable fire rate increase (making the jump from actual 8.6/s to 10/s), but even a +14% boost will do nothing to an 8.7/s weapon (not making the jump and remaining at actual 8.6/s).
- Krieg's Elemental Elation skill highlights this mechanic, as instead of increasing fire rate gradually with a rising number of stacks, there will be jumps in fire rate at certain numbers of stacks, especially with fast-firing weapons.