A real attempt to hit something (as oppose to close your eyes and squeeze) with a simi-auto in d20 is easy, pathfinder revolver rules with appropriate shots/round based on magazine.
Capacity: A firearm's capacity is the number of shots it can hold at one time. When making a full-attack action, you may fire a firearm as many times in a round as you have attacks, up to this limit, unless you can reload the weapon as a swift or free action while making a full-attack action. In the case of early firearms, capacity often indicates the number of barrels a firearm has. In the case of advanced firearms, it typically indicates the number of chambers [or magazine size] the weapon has.Quote: PSRD
Free-action to replace a magazine based on training is simply another example of the rapid reload feat.
For those who simply pull the trigger as fast as they can make a scatter attack, but at the -4 non-proficent penalty (even if they are proficient, because they're not using the weapon in a controlled manner, so a total penalty of -6) but only at the number of targets they have a bab for (so 1 if their bab is under 6, 2 if under 12, etc).
Automatic weapons are treated as above except they actually have the scatter ability so the penalty is only -2. An automatic weapon grants a proficient user the many-shot feat, even if the user doesn't meet the requirements. A user who has the many-shot feat already can fire 4 rounds as a single attack. (Another example of better training and control). Manyshot options are not restricted to the first attack with an automatic firearm.
This isn't a hyper-realistic option, but d20 isn't hyper-realistic, and shouldn't be. However, it covers controlled fire, and spray and pray, for both classifications of weapons discussed.
Feel free to tear it apart for me.