SMGs have a lot going for them. Barrel/overall length is a big issue and their size allows for use in confined spaces like inside vehicles. SMGs can use Rifle/Pistol rounds or hybrid rounds that can due more damage than rifle rounds (and are proving to be just as accurate and sometimes moreso). Lower weight allows for more accessories meaning less fatigue from extended use. SMG builders also experiment with newer ideas (e.g. the equivalent to gyro stabilizing) and built in devices because of lower weight and the higher demand for specialized weapons. Concealable firepower was the mindset behind the SMG (like the Uzi briefcase
http://www.youtube.com/watch?v=e-NCXwPfpDw or the Magpul FMG9
http://www.youtube.com/watch?v=pY2EqFzPzn8). The downside is the range isn't as far as a normal rifle (some SMGs have a greater range than Carbines). Even though they could be used one handed you wouldn't be able to hit anything because of recoil and fatigue. Gun Fu is very Hollywood, but isn't anything more than spray and pray or suppression fire. There is a weapon per hand method, but both weapons are pointing the same way; arm act as the weapons stock (arms locked) and arms crossed at wrists to stabilize. Of course Shadowrun is different boat and futuristic technology/magic could supercede physics (plus last time I checked Shadowrun used caseless ammunition, so recoil is a non-issue)...
Assault Rifles try to get shorter by adding folding stocks (less accuracy) or shorter barrels (lower muzzel velocity, lower accuracy, less kinetic energy/damage per round due to not burning all the powder in the round), or bullpup configuration (largely unpopular due to generations of "standard configurations" = lack of tactile familiarity, some also argue that bullpup = fragile but that's unproven).
Shotguns are known as the great equalizer in the law enforcement community. Even if someone's wearing body armor, a 12 gauge deer slug will not only knock them down, but usually it'll knock them out or daze them thoroughly, which gives more than enough time to subdue the individual. Another thing about shotguns shooting any kind of shot is if you're hit in any extremity, within it's effective range/spread, you're almost guaranteed to lose the limb if you don't die from blood loss or from the invasive nature of the surgery. Even with medical staff on hand, someone shot in the core body will usually die from internal/external bleeding. Shotguns are thoroughly limited when it comes to body armor, they just weren't designed to be used against people. They have been modified nicely to shoot taser rounds, grenades, less-than-lethal rounds, and to become more accurate (deer slugs and rifled barrels).
Every state in the US has different weapon laws. In AZ, where I live, it's not only legal to own automatic weapons, but it's legal to carry them openly or conceal them. Sometimes I feel I'm in a Shadowrun game when I see some of the stuff here. Not to long ago I saw a guy walking down the street carrying a Katana on his hip and a shotgun on his back.