The laws in the United States aren't so much about what the gun looks like (excepting the 'assault weapon' fears, don't get me started) so much as the actual capability for automatic fire; an automatic weapon (or suppressor) is illegal unless you go through the process to get a stamp (it is cheap (the stamp is $200, the price was decided back when a Thompson SMG and some cars cost less than that), but quite time consuming and takes a bit more thorough background checks). Those AR-15s you see slung on peoples' backs? Most are semi-automatic.
The concept of a machine pistol is gray nowadays. Sure, it could look like a Mac-10 or an Uzi or something (Skorpion, perhaps), but if it is semi-automatic, it is legal. In Shadowrun, things are not so gray. Most machine pistols (most automatic weapons short of a machine gun, actually) are R, not F, regardless of fire rate. Therefore, they can be acquired pretty easily. In the dystopic future, they don't care how many bullets your gun can spray, apparently (given automatic fire outside of suppressive fire and certain weapons is basically a waste of ammo anyways, it makes sense).
I still suggest machine pistols for those folks who don't want to pack full-sized heat, but need to put out the lead when things get hairy. Automatics is generally a more flexible skill and you're more likely to be able to pick up a gun from an enemy and use it if you run out of ammo. Shotguns are another option, but getting an automatic shotgun goes straight into the Forbidden zone, where you could get busted worse for having the gun than whatever you did with it.