I find 150g to be too small a batch for heat gun roasting. I tend to roast batches of 300-350g. 200-250g batches work well also. Once you refine a technique, youll find it easier to control the smaller batch size but as a start, Id give 250g a go.
You may also be roasting too hot. There are plenty of guides around on the net to get you started and so you can be aware of exactly what is happening at each stage of the process.