OGC:Adamantine Bullet (5e Equipment)
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Weapon (bullet), uncommon
These bullets are made of adamantine, one of the hardest substances in existence. Attacks with adamantine bullets score critical hits on rolls of 19 or 20 (if you normally score a critical hit on a 19 or lower, then the adamantine bullet scores a critical hit on lower die results on a 1-for-1 basis; for example, if you normally score a critical hit on a 19 or 20 then the adamantine bullet scores one on a roll of 18, 19, or 20).
If you fire an adamantine bullet at a creature wearing adamantine armor, then you only score a critical hit when you normally would (a roll of 20 for most creatures) but the adamantine armor does not downgrade the critical hit.
Adamantine bullets are typically cast in groups of 20.