Xenomorphs don't use a typical hitbox like regular minecraft mobs. Rather, they use a specialised “Dismemberment” system giving every one of their limbs their own hitbox that can be hit.

For Example,

This image displays the unique hitboxes all xenomorphs have. Each part has their own “hp” that is reduced every time it is shot, and once it reaches 0 will cause the limb to fall off. All limbs can fall off.

Some mobs have specific parts of their body that are immune to specific sources of damage. For example, the crusher and queen’s crest are immune to damage from guns added by the AvP : Humans module.

Limbs can be regenerated by Xenomorphs through molting. However, older xenomorphs who have gone through all stages of molting may be unable to regenerate limbs unless they evolve into the next stage if they have one.

Dismembered limbs can be right clicked to drop chitin. The chitin will always be the same chitin as from the strain so an aberrant limb will drop Aberrant Chitin.

As a xenomorph loses its limbs, its max health also decreases causing every limb loss to be more and more dangerous. Whilst a xenomorph can be rendered a “nugget.” The chances of it happening naturally are practically impossible without player intervention.