I’m thinking of handheld weapons vs giant ants or locusts. And yes, I’ve just watched The Beginning of the End and in the past watched both Them! and the Giant Spider Invasion.
Probably as effective as weapons against any large animal. Sure, giant insect wear their skeleton on the outside and are thus “armored” but it’s not like elephant hide is flimsy, either. There are modern handheld weapons capable of shattering bone. You might want to favor some of the man-portable anti-tank weapons, but sure, one (fit) human being can hoist one of those up and aim it. If we can blow up metal-armored tanks with those I expect they’d be plenty effective against giant insects.
A little .22 revolver? Not so much.
Does the fact that arthropods have an open circulatory system make them more or less susceptible to gunshot wounds?
Well, there’s the question of whether or not a giant insect would, by necessity, have to have somehow acquired a closed circulatory system to even exist, vs. somehow being able to scale up an open system.
If confronted by a giant spider and you’re armed with a sword, simply slice off its chelicerae, the two flaps on the face which contain the fangs. Once it’s defanged, its only other weapon is spinning a web to catch you. So finish it off with the sword before it can get you that way.
I was trying not to fight the hypothetical. Obviously, giant insects can’t exist for a variety of reasons, including their circulatory system and the inverse square law, but since the OP posited that they do exist, I’m working based on the assumption that they would be physically identical to real ones, only much bigger.
Well, either way, if you punch a big hole in something living it’s going to be a major problem for the creature regardless of what sort of circulatory system they have.
True, but with an open circulatory system, isn’t everywhere you shoot basically an artery?
Yeah, that’s the real question, isn’t it? If the insect is simply scaled up, it gets an armor plated body, as mentioned above, such that bullets will simply bounce off (except perhaps the eyes?). But such a creature would be so impossibly heavy it couldn’t move, or even really function; so perhaps we must make an accommodation for “realism,” resulting in their having a thinner carapace? Which changes the dynamics a lot.
A spider has claws, but they’re so small that even when scaled up to giant size they could do only as much damage as human fingernails. Spider claws are for climbing, not fighting.
Right, but the original question above is not what the critter can do to you, but what you can do to the critter.
Tactics, Cervaise. One must study its capabilities and weaknesses to fight it effectively.
If only it was that simple. You don’t get to live just because you caused a major problem for the creature. Even lethal hits often take tens of seconds, minutes or even hours to take effect. A hole somewhere may not do even that.
A hole in a spider doesn’t just make it bleed, it destroys the integrity of the hydraulic system required to move its limbs. Shooting a couple holes in a spider will drop it. It will lay motionless, unable to do anything but bleed to death.
Also, even a magically scaled-up spider immune to the cube square law would not have an exoskeleton thick enough to stop an average rifle round. Chitin would need to be like a foot thick to stand a chance. The spider would need to be the size of a building, I think.
Thanks for the spider-specific hydro-scenario.
My reply was prompted by the extreme generality of “something living”, having collected hundreds of game animals with sundry weaponry.
If you shot it in the radiator (in the ass end), you’d probably disable it. Shooting out the red headlights (the eyes) wouldn’t have much effect, IMHO. 
Since Them! was mentioned, I remember clearly that in the first encounter between armed police and one giant ant, the police had some kind of small machine gun (if I remember correctly) that was at first ineffective, and the James Arness character shouted at him to aim for the eyes. Whether the eyes themselves were vulnerable, or something else happened because of aiming for the eyes, is not discussed in the film. (I was wrong, it was 2 handguns, and they aimed for the antennae; then they brought out the “machine gun” and shot at the head.)
After that, they used flame throwers, which were portrayed as being very effective, in that the ants caught fire (or the burning medium stuck to the ants and burned them).
I have secretly hoped that they would find that the scientist guy was wrong and that another queen had set off to establish a new nest somewhere unknown, so that there could be a sequel called Them Too!
Don’t try that with THEM.
Not the eyes.
The antennae.
Yes, see the correction in the first paragraph. Also it was 2 handguns shooting at the antennae, and then a “machine gun” shooting at the head.