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As previously noted, your ammo (in combination with your short barrel) is spitting powder that is still burning.
A faster burning powder will reduce or eliminate the amount of unburned powder. Powder with smaller granules is also probably helpful as the duration of burn after leaving the barrel should be less.
My indoor range had problems with steel case Russian ammo igniting fires. The residue from paper targets and cardboard backers makes for a fluffy and easy to ignite combustible material.
Gun + ammo combinations that spit too much burning powder are a fire hazard at both ranges and if shooting outdoors when conditions are dry. It would be nice if the range was kept cleaner, but I would not expect it to be "fireproof".
I would try to arrange a situation to safely get rid of the rest of the problem ammo and then never use that load again.
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