basically grunge is a cleaned up version of pet rock. a slow sludgy offshoot of punk espoused by flipper, negative trend, and the frantic animals. It became grunge when the melvins combined flipper with sabbath and started playing detuned power chords instead of just feeding back and warbling aimlessly around some basic barr. so by that little root, nirvana is punk. although, if you listen to some of their weirder unreleased stuff, they do tend to wander into that weird little universe inhabited by primus, frank zappa, SWANS, and sonic youth.
they all have punk roots too: Dave was in a DC hardcore band called Scream. Krist was in the Jello Biafra fronted No WTO Combo, Pat Smear was in the Germs.
is that enough? if your still gonna be stubborn and say that all punk is "hardcore" and all songs that are punk sound like the circle jerks or minor threat, here's a list of songs that could be considered "hardcore":
originals: (new wave) Polly, Territorial Pissings, Downer, Curmudgeon, Oh, The guilt
covers: D7 (wipers), Return of the Rat (wipers), Yes, she is my skinhead girl (stooges), molly's lips (vaselines), Son of a gun (vaselines), Sacrifice (flipper), sex bomb (flipper), the way of the world (flipper)