If I was trying to get a propane tank to explode with a tracer, I'd shoot nearer the top of the tank, in the (hopefully) vapor portion above the liquid line. It's the vapor that burns.
Where did the Mythbuster guys shoot their tank (s)?
"Always place your clothes and weapons where you can find them in the dark"
"Understand that the enemy is not the enemy in his own eyes ;this may offer you an opportunity to make him your friend. If not, you can kill him without hate, and quickly."