I can vouch for an almost empty 1lb can of propane. We set it up with a small fire behind it, shot a 7.62 and HOLY CRAP what a fireball. Not a quick one either, it lasted a good 10 seconds. We thought the grass/weeds to the side was going to ignite.
So if you shoot a *full* can, be absolutely sure nothing is flammable around it and you're about 50 yards out.