Morally speaking, though, there is only one action, and the Mens Rea usually concern the action taken as a whole, and not each separate compoment (though it's certainly possible to have the deliberate intent of breaking two laws, as opposed to the intent of taking one action which happen to involve components that violate several law). If you only intend to carry out one action, then morally speaking there is but one criminal action; one crime.
However, this one criminal action then has to be characterized in legal terms, which essentially involve taking the action and showing in what ways it broke the law. This can allow for a number of combinations. For example, in your example above, perhaps I thought based on credible information that the gun, while realistic looking, would only fire blank shots; in this case in some jurisdictions the charges may end up being discharging a gun at someone and involuntary manslaughter, which is a different combination than discharging a gun at someone and second-degree murder (in the heat of the moment), and still a different combination than discharging a gun at someone and first-degree murder (premeditated), and still different from discharging a gun at someone and injuring them, and discharging a weapon at someone and not even hitting. Since in some cases it's entirely possible to discharge a weapon in someone's direction with no intent to kill, "attempted murder" does not fit.
Thus why to characterize each crime the legal system use building bricks - charges - to assemble a description that fit the incident. How thorough that description is usually depend on the case and prosecutor, and plea bargain can often be used to get some less-damaging building bricks swapped in for more-damaging ones. (For example, in a murder trial, agreeing that you were really careless and took really dumb actions, while the prosecution agree that you weren't really trying to *kill*, thus resulting in you pleading guilty to involuntary manslaughter rather than first- or second- degree murder).