"B" is for "Burstworm"

Burstworm

No. Enc.:  2d4 (3d12)
Alignment:  Neutral
Movement:  3' (1')
Armor Class:  5
Hit Dice:  1d4 hit points
Attacks:  1 (bite)
Damage:  1d3
Save:  L1
Morale:  12
Hoard Class:  None
XP:  7

Burstworms are bizarre, 3"-6" long annelids that feed off of gunpowder and volatile compounds.  They infest Ancient armories, bunkers, and weapons caches, and nest in rigid, cylindrical structures...meaning firearm barrels are their ideal habitats.

A burstworm is a patient hunter, and usually waits for someone to pick up the gun it calls home.  Then, like an eel of old, it lunges forth with snapping fangs.  With an attack roll of a natural 20, the burstworm bites off its victim's finger, thumb, or nose (whichever is closest).

Burstworms are even more dangerous dead than alive.  When killed, a burstworm explodes, doing 1d8 damage to all creatures and items in a 5' radius.  Other burstworms are not immune to this effect.  Entire scavenging parties have been wiped out when one specimen detonated, igniting a chain reaction throughout a worm- and munitions-filled stockpile...!

There is evidence that a burstworm's size is directly related to the dimensions of its enclosure, as burstworms that live in handguns are usually smaller than those that live in rifles.  Theoretically, there could be burstworms of freakishly massive size dwelling in tank turrets, torpedo tubes, or even missile silos....

Mutations:  Aberrant Form ("Combustible Body")