The enemies feel like bullet sponge because the demo is an early level. Your initial weapons stink, especially the pirates' AK47 (They are very cheap weapons). The pistol and shotgun are more effective. As you progress, you'll get better ones. I seldom use the AK because it's arguably the weakest weapon in the game. Sometimes it comes in handy because I need to spray fire (No time to take better aim). In comparison, it takes about 1-5 pistol shots to kill a pirate -- depending on where you hit them..
Huh, i found the AK47 just fine, certainly better than the rubbish pistol you start with.