Well Web-14 is about the same as a PG-13 movie rating, and most action movies fit in under that one. I suppose the question is how often you see insane amounts of violence, and to what degree you'd take it. Also since we're in a drawn medium, and it's less realistic, it takes things down a notch from reality.
There's also the 3 strikes rule. Violence, Language, and Sex/nudity. It takes three for a movie to get an R rating. Though there's factors like setting, mood, and the interplay of events. For instance, violence in the middle of a battle is going to be taken better than cold blooded murder or torture. My own comic I rate Web-14. It has some violence, most of which is gunfire, though there's one off panel axe murder, however most of it is just the occasional ass kicking, most frequently courtesy of Nailbat. Nothing truly gory, aside from one decapitation, which also happened off-panel, though the head was shown.
Sex and nudity isn't an issue, since most of my characters CAN'T get naked. They still have a layer of all concealing fur. Some of them don't even wear pants. I'm not sure if that's going to be an issue in your comic. If you keep the nudity down, you can probably crank the violence up a notch. It seems to be a mix of the two that really puts people off.
Language I personally keep to a minimum, by just swapping out local profanity, for either fictional profanity or the occasional British swear like "bloody" and "bugger," which most of the audience interprets as rude but not offensive. Though these days nothing but frequent and consistent F-bombs get anyone's attention. Of course a violent comic can come off unrealistic if people don't swear at all. In real life people swear. It just happens, usually when we're angry. I'd say just swear as much as you would in real life, and it shouldn't be too egregious.