I have a lot of experience of those propane cannons - all operated by neighbours. They were very popular in the UK. It seems to me that they are not effective after the first few days. The birds seem to get used to them and often do not even take off, although mostly I have noticed they get a fright, fly up, and then settle down to feeding again.
I do not know if it works but most people around here use CDs strung in bushes or on poles. Another TBNer, Bill Spurlock, makes his own scarers. See his blog sunnyslopesorchard.blogspot and look for his thread about them a while back.
Slightly off-topic, but not entirely because it involves killing crop eaters, I hope you do not mind my next post to dragoneggs.