CountryBoy19
Grandmaster
I attempted to knock the bucket over and get it upside down to hopefully get the oxygen out of the bucket to put the fire out which I succeeded.
What I think happened was when I fired that last shot a hot .22 brass fell into the bucket and ignited all the unspent gunpowder inside the bucket.
These 2 sentences don't make sense. If you were able to smother the fire by removing oxygen from the equation then the fuel source was not unburned gun-powder. Gun-powder contains it's own oxidizer and does not need any outside oxygen to burn. What is more likely is that there may have been a minute amount of unburned powder in the bucket which in-turn ignited the dust. Despite what commons sense says, often times "dust" is not only combustible, but can actually cause explosions if it's airborn. That is because dust is often very fine particles of a combustile material (ie wood dust, grain dust, etc). In the bucket like it was it is much harder to ignite, but the presence of a bit of unburned powder would have helped that.
I also doubt hot brass started it, more likely it was a spark or burning ember inside a piece of brass that fell into the bucket.
Either way, glad you weren't injured and were able to put it out. Hopefuly the insight I provided can help people understand how this probably happened and how to prevent it in the future...