How do I open a security van in GTA V?

Once you unlock sticky bombs you can apply one to the back of the van and blow it. It will open the doors without destroying the contents. It is the easiest and most efficient method.

It's very simple, take out your shotgun and fire in between the two back doors. After 4 or 5 shots, the doors will pop open and a briefcase will drop to the ground. I have been getting different amounts, last time it was $20,000 in it then if you have the cops after you lose them.

I went to Lago Zancudo, there is a gas station where I fortunately found a tanker (what trucks use to carry gas). I parked the security van next to it and I shot the tanker from a distance. Four shots later it blew up and open the backdoor, dropping a brief case with $7,000!!!!