Well..."metalcore" is pretty much just metal bands playing hardcore shows most of the time. Really it's more of a social divide than anything else...kids who are into metal and hardcore, and these days most of them really only listen to metal because most of these very metal influenced bands just call themselves hardcore bands. It's a bit of a convoluted thing...although a lot of people I know in "metalcore" bands are just metalheads and hardcore kids getting together and starting bands. I used to play in a band that was pretty much just brutal death metal with lots of chugga chugga type breakdowns, lots of double harmony riffs and Death-esque parts with a singer who had one of those low growls that sounde like he was possesed by satan. That band was pretty much a mix of metal heads and punks.
These days, musical styles are so crossed over and meshed, the only real difference between a lot of bands is how they dress, what kinds of haircuts they have, and what label they slap on themselves. Look at crust and grind bands...some of them are straight out gutter punks, but you couldn't pick them out from long haired brutal metal/grind band like Benumb any day of the week.