Step one, hijack a small compact vehicle.
Step two, drive it into the ammunation doors, it'll probably get stuck/won't fit.
Step three, get out and throw a sticky on the car.
Step four, hijack another car if you don't want to get your personal one dented.
Step five, Drive into the car parked against the door repeatedly until you manage to shove it into the store sideways.
Step six, drive away, step out and detonate.
Step Seven, enjoy.