Edit: Here's a more interesting infinite combo, still with 11 cards:
Play Darksteel Citadel, tap for (1)
Play Ornithopter for 0
Play Mox Opal, tap for U
Play Mox Opal sacrificing the other one, tap for U
Play Skill Borrower for 1UU, reveal Krark-Clan Ironworks
Sacrifice Darksteel Citadel, Ornithopter, and Mox Opal to Skill Borrower for (6)
Play Mimic Vat for 3
Play Ichor Wellspring for 2, but hold priority...
...Sacrifice the Ichor Wellspring to Skill Borrower for (2) and hold priority...
...Sacrifice Skill Borrower to itself for (2), imprinting onto Mimic Vat
...Tap Mimic Vat for 3, creating a token copy of Skill Borrower with haste
...Sacrifice Mimic Vat to the Skill Borrower token for (2)
...release priority, drawing Krark-Clan Ironworks and Voltaic Construct, revealing Dreamstone Hedron
Tap the Skill Borrower token for (3)
Play Voltaic Construct for 4
Pay 2 to untap the Skill Borrower token with Voltaic Construct, then tap it again for (3)
Repeat until you have infinite mana
Tap and sacrifice the Skill Borrower token for 3, drawing Dreamstone Hedron, Goblin Cannon, and ???
Play Goblin Cannon for 4
Shoot your opponent for infinite with Goblin Cannon
Turn 1 First Main Phase on the Play - Infinite Damage, 11 cards
Play Darksteel Citadel, tap for (1)
Play Ornithopter for 0
Play Mox Opal, tap for U
Play Mox Opal sacrificing the other one, tap for U
Play Mox Opal sacrificing the other one, tap for U
Play Krark-Clan Ironworks for 1UUU
Sacrifice Darksteel Citadel, Ornithopter, and Mox Opal to Krark-Clan Ironworks for (6)
Play Ichor Wellspring for 2, drawing a card
Sacrifice Ichor Wellspring to Krark-Clan Ironworks, drawing a card, gaining (2)
Play Myr Retriever for 2
Sacrifice Myr Retriever to Krark-Clan Ironworks, returning Ichor Wellspring, gaining (2)
Play Ichor Wellspring for 2, drawing a card
Sacrifice Ichor Wellspring to Krark-Clan Ironworks, drawing a card, gaining (2) (6 mana, 3 cards)
Play Locket of Yesterdays for 1
Play Goblin Cannon for 4
Play Myr Retriever for 1
Sacrifice Myr Retriever to Krark-Clan Ironworks returning Myr Retriever, play it, repeat until you have infinite mana, then blow Goblin Cannon for infinite damage
Older Solutions:
Turn 2 Combat on the Play - 11 infect.
Turn 1:
Play Inkmoth Nexus, tap for (1)
Play Ornithopter for 0
Play Springleaf Drum for 1, tap it and Ornithopter for U
Play Mox Opal, tap for U
Play Arcbound Ravager for UU (1 counter)
Sac Mox Opal to Ravager (2 counters)
Play Mox Opal, tap for U
Play Adventuring Gear for U
Turn 2:
Tap Springleaf Drum and Ornithopter for U
Activate Inkmoth Nexus for U
Tap Mox Opal for U
Equip Adventuring Gear to Nexus for U
Play Polluted Delta, fetch for Island (+4/+4 to Nexus)
Sac Mox Opal, Springleaf Drum, Ornithopter, and Adventuring Gear to Ravager (6 counters)
Sac Ravager to itself, transfer all 6 counters to Inkmoth Nexus
Attack with Inkmoth Nexus for 11 Infect