What assault rifles can police use?

Whatever their agencies authorize for use. Most comonly used "cruiser rifles" are .308's - .223's - and .30 cal carbines. SWAT-type units use different types according to whatever their department happened to purchase. Most are of the short barreled full-auto type weapons (i.e.: UZI's - H&K Mp-5's - etc). This is by no means a full list of law enforcement weapons that can be found in use. The most common "long" gun carried by the police is the good old 12 guage shotgun.