Depleted uranium also superheats when it hits the target, which is what I assumed you meant when you said it melts through tank armor, Drall.

But getting back to the flame character, most bullets are made of lead with a copper jacket, not steel, and while lead has a significantly lower melting point than steel (621.5 degrees fahrenheit, which is cooler than your average campfire), copper is sitting pretty at almost 2,000 degrees fahrenheit, which is still too high for a completely passive ability. Even if he could passively melt bullets though, he'd still have to worry about the molten metal since melting an object wouldn't eliminate its momentum, and a supersonic glob of melted anything is still going to hurt. A lot.

Edit: Looks like QT beat me to the punch on my first point.