Like the katana, the SMG isn't necessarily the best or most powerful firearm in Dead Rising. But it makes our survival guide thanks to its generous 100-bullet clip and quick rate of fire. That helps keep the bad guys at bay, even if you'll run through your ammo in a matter of seconds.
The SMG is another semi-powerful weapon you'll be able to find almost immediately upon entering the mall. Once you've come through the warehouse, turn right and head up the stairs. On the first landing, jump toward the blue awning to the right. Run along this awning to its end and you'll find the SMG.
If you happen to be escorting a survivor and running low on ammo for your SMG, here's a tip: give your nearly-empty noisemaker to a nearby survivor by standing next to them and pressing the B button. Now give them another weapon, forcing them to drop the original gun. When you pick up the gun again, ta da - it's reloaded.