ORIGINAL: Briman
Hot bullet? no. The air would cool the bullet down if anything.
Singed paper would be brown in color not black or gray.
My best guess would be powder residue (graphite) on the bullet rubbing off on paper.
Gas cans or propane cylinders won't explode when hit by bullets, unless you get all of your information from Sillywood. 
Kinda my thoughts too, Ive seen both a steel gas tank out of a chevy truck about 1/4 full of gasoline and a coleman propane cylinder both shot. the gas tank was shot with 15 rounds from a 9mm, my buddy was hoping it would explode......... it never did. The propane cylinder was shot with a 44 magnum it too did not explode into a massive fireball like it does in hollywood aka sillywood. The propane cylinder did shoot off like a rocket about 50 feet into the air spinning all sorts of crazy though.