[Pollster1]

used a blow torch to light a machine gun bullet

I would have guessed that a bullet would not burn, even under the heat of a blowtorch. Maybe this bullet was made from a special kind of lead, or perhaps is was an incendiary armor-piercing cop-killer green-tip special bullet for an AK-47 pump-action 12 gauge fully automatic Glock machine gun.

I wish journalists knew at least a little about the weapons and ammo that they hate so much.

[eastforker]

I bet it was an API bullet with tracer.

[muir_redwoods]

It was a tracer round coated with whatever they use now instead of phosphor

[Hulka]

I’m thinking it was one of these:
http://mcadams.posc.mu.edu/blog/afp_Bullet.htm