>>213966047
My headcanon is that the pressurized blood would repel projectile weapons kind of like how reactive armour works to divert AT weapons from penetrating the hull and killing everyone inside. The bullet pierces the exoskeleton, the blood immediately explodes outward with force, the bullet is either knocked away on glancing shots or most of its energy is absorbed trying to fight against the force of the blood. The pulse rifle rounds in Aliens were High Explosive which means that they aren't dependent on penetration depth or kinetic energy for effect, the payload detonating inside was enough to burst their carapaces open. With conventional bullets you'd either need rounds with a lot of mass to penetrate deep or sheer volume of fire in the hopes you can bleed off the pressurized exterior and start getting into the gooey insides.