Cheapest priced copy (Onslaught): $0.49 at Card Kingdom, near mint, their price list as of .
Doom Cannon 
Artifact · Rare · Onslaught · #307
As this artifact enters, choose a creature type.
{3}, {T}, Sacrifice a creature of the chosen type: This artifact deals 3 damage to any target.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper