To anyone bringing up the grappling harpoon gun in ALIEN as some kind of proof that Xenos are weak and easy to take down: Yeah, the grappling harpoon lodges itself between its ribs, and although there's is what looks like a light spray of acid blood coming out once it's hit, we see no more blood coming out after the initial hit (...so one could therefore argue that the harpoon only "scratched" the Xeno as it hit and literally lodged itself in the rib structure, rather than actually penetrating it Xenomorph. Remember, the harpoon has grapplers that unfold once it hits and the Xeno has an accentuated, protruding, open rib structure, so the harpoon lodging itself without truly penetrating its chitin is not as much of a fluke as it seems. If the harpoon had indeed truly penetrated and wounded the Xeno, the wound would have been large and messy, gushing blood, due to the prongs/grapplers and large entry hole, and we would've seen lots of bleeding...).
In fact, the Xeno seems completely unphased by the harpoon other than the obvious fact that it's attached to its wire. In fact, the Xeno makes its way back to the shuttle after Ripley closes the door and then survives a looong spaceship jet blast, which has seemingly no effect on it either other than it being pushed away from the shuttle, propelling it into space. So according to that scene flamethrowers, incendiary weapons, plasma and the blast of an explosion (not counting the shrapnel of course) should have limited to no effect on Xenomorphs. Actually, the Runner's lead bath also confirms its resistance towards immense heat, fire and pressure.
Back to the harpoon: Also, had it truly punched through the Xeno's carapace/hide, the acid blood would've corroded the harpoon (and the wire) quickly, having it snap and break, and the Xeno would've been released in no time. To it seems more likely that the harpoon lodged itself inside the outer intricacy of the Xenomorph's exoskeletal rib structure as the prongs/grapplers sprung right after impact. The initial tiny blood spray we see comes from scratching the outer layer/membrane covering the creature.
Anyways, that's been my interpretation since the first time I watched ALIEN.