Voltaic Key, Rings of Brighthearth, and Astral Cornucopia

Prerequisites

• Voltaic Key on the battlefield
• Rings of Brighthearth on the battlefield
• Astral Cornucopia on the battlefield
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Steps

1. Activate Astral Cornucopia by tapping it, adding at least four mana of any color
2. Activate Voltaic Key by paying and tapping it, targeting itself
3. Rings of Brighthearth triggers, causing you to pay to copy Voltaic Key's ability, targeting Astral Cornucopia
4. Resolve both Voltaic Key abilities, untapping it and Astral Cornucopia
5. Repeat

Results

• Infinite colored mana
• Infinite untap of artifacts you control
• Infinite mana artifacts you control can produce