No, "assault rifles" are not perfectly legal...
Bzzzzzt. Wrong.... The only rifles that are restricted to own are automatic rifles. ie: machine guns.
Bzzzzzt. Wrong. An actual "assault rifle" is a select-file (i.e., can be set for auto or semi-automatic operation) rifle of intermediate power. They are automatic weapons, and as such heavily restricted.
"Assault rifle" is not to be confused with "assault weapon", which is the sort of "ugly gun" you're speaking of. The term "assault weapon" seems to have been a deliberate coinage by a prohibitionist to confuse scary-looking semi-automatic rifles with actual military select-file assault rifles.
"Assault rifle" is a meaningful term. "Assault weapon" is not.