Using Wobbles' Quick-Draw mechanic:
Quick-Draw (Reveal this card from your hand until it leaves your hand: Target creature gets +2/+0 or -2/-0 until end of turn.)
In order to get the critical density of Quick-Draw cards required for the back and forth shots of a firefight the mechanic has to show up everywhere, so this submission includes commons of all five colors that either have Quick-Draw or show off the interactions it creates. Time for a Duel at high noon!
*Comments marked by asterisks*
Breakneck Speed
R
Sorcery (C)
Target creature gains haste until end of turn.
Quick-Draw (Reveal this card from your hand until it leaves your hand: Target creature gets +2/+0 or -2/-0 until end of turn.)
[Sometimes even the Pony Express ain't fast enough.]
*Pumping your hasty attacker is great, but using this earlier will let your opponent know to leave back blockers.*
Perplexing Sight
2BB
Sorcery (C)
Target player discards two cards.
Quick-Draw (Reveal this card from your hand until it leaves your hand: Target creature gets +2/+0 or -2/-0 until end of turn.)
[What in tarnation?]
*Similarly, using this early will tip off your opponent to sandbag lands.*
Reevaluate
2U
Instant (C)
Draw three cards, then put two cards from your hand on top of your library in any order.
[After a moment Kren decided that maybe three on two weren't the best odds after all.]
*Reuse Quick-Draws again and again!*
Juvenile Rattler
G
Creature-Snake (C)
Deathtouch
[The young’uns are the deadliest. 'cause they don't know how to hold venom in.]
1/1
*Deathtouch creatures usually hold back attacks, but with Quick-Draw they instead make for tense blocking situations.*
Expert Wrangler
2W
Creature-Human Scout (C)
First Strike
Immobilize (Whenever Expert Wrangler deals combat damage to a creature, tap that creature and it doesn’t untap during its controller’s next untap step.)
2/1
*First Strikers are similarly less likely to cause board stalls. Immobilize is a reasonable solution to the blue common keyword dilemma and fits here too.*
Viashino Warrior [reprint]
3R
Creature-Viashino Warrior (C)
[He knows the area like the back of his hand, er, claw.]
4/2
*4/2s are usually dorky creatures that trade for cheaper enemies, but when Grizzly Bears start changing into 0/2, this awkward P/T combination suddenly denotes an undercosted beater.*