I wanted a wild-west themed burn card, but since it can't be reprinted in a non-guns set, I looked for a CMC/damage combination that didn't seem to exist but might not get reprinted anyway.

Might tread on the toes of Spire Barrage, if so, I can change that one

4 damage is a weird spot on the costing scale of burn. You get it to just creatures for 1 mana (Flame Slash, albeit sorcery speed), or to players for 3-with-upside (Flames of the Blood Hand), but to your choice of creature or player is either 4-5 mana (Lightning Blast) or fiddly to cast (Flame Javelin). (5 damage is 5 to a player - Lava Axe - or 6 to anything - Explosive Impact.)

Yeah, I gave up finding a consistent metric for 4-damage spells. I think it's very skewed because (a.) they try to avoid printing strictly-better or strictly-worse spells and (b.) they try to avoid printing a critical mass of good burn spells in standard or (maybe) modern.

So I think 4-damage spells tend to be overcosted, so there's room for three damage spells to exist. Hence, I costed this aggressively, since it will never be in standard. But I think reasonably -- there's lots of 3-mana 3-or-5 damage spells at common, even though flame javelin et al were worse and uncommon. But it would also be perfectly good at 4 (or 5) mana, so I can tweak it later.