For example, "assault weapons" are a made-up category of weapons that is based solely on cosmetic features that make them look like the fully automatic weapons used by the military. Banning them leaves other rifles that are functionally identical in their lethality and rate of fire completely legal. Moreover, far more powerful hunting rifles are left untouched by the law, as are shotguns. This is simply irrational and therefore unconstitutional.
The same can be said for New York's law limiting handguns to seven rounds, while allowing both active and retired police officers to keep their handguns that hold up to 15 rounds. If retired cops need 15 rounds to effectively protect themselves and others, then so do other citizens. Arbitrarily discriminating among Americans in this way is irrational and unconstitutional.