The SMG is a less tactical weapon than the pump shotgun, but not much so. You can fire the entire clip in one long burst, sweeping it back and forth across the room, but this is rarely advisable. The gun is the most useful when used in short bursts - aim for the head, and you will get a huge number of kills per clip. This is very accurate at range if used in single shots or short bursts, so don't be afraid to start firing from far away. Stay calm and resist the urge to spray and pray, and this gun will treat you well. In close combat, the SMG is still useful, but less so than the shotgun. Stand behind your allies with shotguns, or kneel and let them deal with the threat from behind.
The Assault Rifle is an extremely powerful weapon. A hit almost anywhere on an infected with this gun will kill or cripple them, so make good use of it. Even more so than the SMG, this gun is excellent at range, so take the time to pick off distant or isolated infected. What makes this weapon so fantastic is that it is powerful at close range too - with bursts or on full automatic, infected will have trouble closing with you as long as you have bullets to fire with. Be careful though, the spray on this weapon can get very wide, so make sure that your allies aren't in range when you start to open up.