My experience is that, even with a (pretty close to) perfect propane-air mixture and center ignition, the existance of a small hole (say, 1.4" in a 1 gallon jug) will severely slow the combustion.
For instance, if one takes a 1 gallon milk-jug, rigs up a center ignition point, but doesn't screw the lid on very tightly... it pops off, and you get a "Pwoosh".
Meanwhile, if the lid is on tightly, it goes "woBong". Loudly.
I'm not entirely certain <i>why</i> this happens.
Anyway, that's my geuss: the lid is popping off pretty early, creating the same conditions as in my milk jugs. No pressure... no projectile movement.