Take, for example, the weapon pictured above. That is an FN P90. This weapon is pretty standard for military and counter terrorist forces throughout the world today, and is very popular in movies, due in large part to it's futuristic look and wide availablity. But there is a problem with how they are used, let me show you:
Watch the video, and pay attention to the number of seconds she is firing on full automatic for. By my count it's about 6 seconds. But with that firearm she only has a 50 round clip, at 900 rounds per minute, the clip would be empty in 3.3 seconds! This is the problem that I'm talking about. Even in movies where they remember to reload at all, they forget that most assault rifles and sub machine guns will run out of ammunition in less than 5 seconds with full automatic fire. Just casually glancing through wikipedia, I couldn't find a single assault rifle or SMG with more than 10 seconds of continuous fire on full auto.
So remember, if your protagonists or antagonists use their weapons at full auto, they'll probably need to reload every 3-5 seconds.