A Lab can chomp; according to a poster that I saw in a vet's office, the Lab is the number one breed for biting.
We have a black Lab who will bite; whenever people come over, we have to tie him up to keep him from biting.
Mine is very pushy about getting a hold on you if you don't pay attention to her, but she never bites down, she just holds gently.
It may have something to do with breeding a soft mouth for the birds. I have noticed that my Lab's canines are short and blunt and very low in the gum line - not the teeth of a serious biter. Not that she couldn't put a hurt on somebody if she wanted to -- but she doesn't want to.
That said, I have seen quite a number of dog-aggressive Labs and a few human-aggressive ones. The field Labs are usually the culprits because they are wound VERY tight.