In America, something like 35% of households have at least one firearm, though this is closer to 20 or 25% in the Northeast. Nevertheless, it would be more realistic to regularly find the occasional handgun, shotgun, or rifle in a drawer or gun safe.
In order to balance this, gun safes would usually be locked and take considerable effort to get into, probably requiring a screwdriver and some time to open (Gun safes are often less safe then is popularly believed), and it would also be realistic to have weapons with full-auto capability restricted to military spawns, bunkers, outposts, and the occasional larger gun store or civilian collection. Fully automatic weapons are extremely expensive for civilians and require jumping through a large number of hoops, acquiring a class 3 license, paying the $200 dollar fee for each weapon purchased, and each fully automatic firearm can cost upwards of $10,000.
In addition, M4 carbines and M16 rifles issued to the military are generally not capable of fully automatic fire, instead having 3-round burst capability with the exception of certain rare variants usually carried by special forces.
