• Grants a heart container.

• Chief Master - If the player has [Alien Sidearm] and any two Master Rounds, the [Alien Sidearm] has a much larger magazine and cannot be charged, but becomes fully automatic and the final two shots in the magazine are fully-charged shots.
• Master's Chambers - Each Master Round the player carries permanently unlocks the [Chamber Gun] 's mode of its respective floor. Reloading the [Chamber Gun] with a full magazine cycles through the unlocked modes.
• Master Rounds are one of the necessary materials to unlock shortcuts . When they are used for this purpose, the Tinker will take away one Master Round, and therefore remove a heart container from the player.
• Master Rounds will not appear if the player takes any kind of damage during a boss fight, including falling into pits and other kinds of damage not inflicted by the boss itself. Master Rounds will also not appear if the player takes damage during a boss fight, and then restarts the boss fight using the [Escape Rope] or [Teleporter Prototype] .
• Master Rounds can be dropped, which will remove a heart container. When they are picked up after being dropped, they will add an empty heart container.
• Master Rounds give one piece of armor to The Robot Dropping a Master Round as The Robot will not remove the armor. Picking it up again will not get any new armor.
• Master Rounds can be sold to the Sell Creep for 3 ; if sold in a special room in the Hollow along with two other Items , it will open the entrance to the R&G Dept. .
• Master Rounds can't be stolen by the Resourceful Rat .
• Acquiring five Master Rounds in a single run unlocks the Lead God Achievement and [Super Hot Watch] . The achievement can still be unlocked via Boss Rush or by acquiring duplicate Master Rounds with [Clone] . For example, if the player gets four Master Rounds, then dies with [Clone] and respawns in the Keep of the Lead Lord, the achievement will be unlocked as soon as they get another Master Round.
• It is not necessary to pick up the Master Rounds to unlock Lead God (although there is no reason not to pick them up). The achievement will still unlock if the player beats the five bosses flawlessly without picking up the Master Rounds that they drop.
• Killing a secret boss without taking damage counts toward unlocking the achievement.
• There exists an unused sixth Master Round , likely as a prize for defeating the Lich without taking any damage.
• In co-op, the Master Round only affects the player who is holding it.

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