After having run my Axeman-Anderson 130-M (similar to the Coalgun) since the fall of 2007 and having had minimal puff back issues and no explosions, I have this season just run into several sizable puff backs that I would have to characterize as true explosions. Unlike my first experience, they did cause loud noises. They also allowed sulfur odor to enter the boiler room and adjacent room.
On two occasions the "combustion bump" as Axeman calls them in their manual, was enough to set off my smoke alarm at the time of the puffback. Not sure why. It went off simultaneously with the bump on two occasions. I witnessed one directly and it scared the bejesus out of me. I was lucky to have just moved away from the inspection port seconds earlier! At no time did these puff backs set off the CO detector! I assume they had burned up the CO in the minor explosion.
The first time I experienced the problem was at the very beginning of using the AA 130 in 2007. After increasing the amount of air to the boiler room, I no longer had the minor smelly puff backs. Nine years later, nothing has changed about venting, chimney, settings on my system operation. I did remove the baro about three or four years ago because I felt is was unnecessary as we did not have excessive winds and the Type M baro was expensive and highly susceptible to rust. Also, the 5 to 6" jump up in vent pipe size before the chimney was a velocity killer for fly ash and soot which built up too high each year. By removing the baro and switching to a 5" pipe run directly to the vertical portion of the chimney, the velocity is maintained and I don't have to clean the horizontal pipe at mid season of fly ash. My coal supplier has been the same for two years but I think this year his source was different and I have yet to query him on this. I will have to report back. I haven't paid much attention to where my coal comes from because the Axeman hasn't been picky. I have burned pea Reading and Superior and pea and buck from Superior and Blashak, but this last load I believe came from another mine/source, possibly the new Lehigh Coal.
While I have solved the combustion bump problem, I still have had a couple quiet but smelly puff backs since I installed, NOT a Freddy style ANTI-BOOM device, but rather a mikeandgerry variation on the paper clip trick, i.e. a paper clip bent into an acute angle and placed in the site port tube to keep the site port door from closing. I used two hair pin style clips like you would use on a clevis pin on each side of the site port door to hold it open a little bit more and a little more visibly and reliably. Since I am using buck, I also have the 3/4 " plug out of the site port door for additional air-over-fire as recommended by Axeman Anderson. Again, no more explosions but a couple mildly smelly puff backs.