From studying the Ranged Weapons table in d20 Modern, it's obvious that the HK G3 7.62mm assault rifle (shouldn't that be battle rifle?) is the best of the "Longarms", for use in just about any combat situation one might encounter in a d20 Modern adventure. The HK G3 has the best range and damage among longarms, and it can fire semi- or fully-automatic. Only a player who puts style and roleplaying above combat potential/effectiveness would want to settle for anything less than a HK G3, for his character. Yet when it comes to longarms in real-life, a 7.62mm battle rifle is seldom the first choice among commandos, special forces, SWAT teams, etc.
If a d20 Modern adventure involved the player characters going into a labyrinth-like office complex, shouldn't they prefer using submachineguns over assault rifles, and especially over battle rifles? If another adventure involved the player character going into a cramped sewer system or clearing out a Vietnam-like tunnel and bunker network, shouldn't they prefer using pistols or sawed-off shotguns over anything else? Yet the d20 Modern rules for firearms are not conducive for that sort of thing, since in d20 Modern combat it's all about range, damage, and rate of fire – and nothing else.
Not that I'm advocating complex rules involving recoil and weapon speed/size, here. But surely, we can use something a little more detailed than what d20 Modern is giving us.