‘Semi-auto’ would generally always be an option and behave like firing normally does now. One aimed shot.
‘Short bursts’ might require more time and a handling check (and/or a check based on a weapon’s quality) to fire as many as intended, but would not require the firearm to be automatic - you’re just shooting it faster, albeit not as fast as an actual automatic. Individual firearm skills could increase accuracy and expand your options, from ‘fire wildly’ at level 0 to ‘fire precisely six shots’ at level (whatever), but there would need to be a way to keep it from being too fiddly. I recall individual firearm skills affecting reload times but don’t remember if it also affects the time it takes to shoot - if so, this could be reflected in your maximum number of shots available per burst.
Then ‘full auto’, for times when you don’t mind spending up to an entire clip, that might work similarly to how a short burst works for non-automatics, except faster and more difficult. Bridging between full-auto and small burst-fire would be those automatics with a three-round burst function, skipping the fire control step and shooting precisely three bullets.
Each setting trades fire speed for accuracy and extra checks for things going wrong. Higher fire rates should have a dramatic effect on accuracy, and skill levels themselves should have a dramatic effect on this penalty. Certain firearms might not have access to certain settings, e.g. some SMGs either fire automatically or one at a time, LMGs have no semi-auto, and most pistols can’t empty a clip in one turn.
No matter the skill level, some weapons would have a cap on how fast they would fire, 100 moves being six seconds. We might be well-served making any speed boosts also increase the base fire time of a weapon, or else let people who are faster than normal warp time and space to let firearms shoot above their rpm.
EDIT: ‘time to shoot’ in most cases means time spent acquiring the target, aiming for weak spots, managing recoil, picking your nose, etc. I don’t think any weapon should have an in-game rate of fire that reflects its maximum theoretical speed while maintaining any accuracy.
