I do the majority of my training in the basement using green gas and I have never had a problem. I have also used propane but stopped due to the smell. I am not saying that there is zero risk, but with the amount of gas released from 10-12 magazines over a 45 minute period or so should be fine. This also is dependent on the size of the space being used to train.
Closing the door to the furnace room should be helpful as well. I will post some photos of my training area in the next few days.