Plenty of fireworks readily available in Japan to provide a source for gun powder.
Based on the video, the first barrel nay have misfired/malfunctioned (noise, smoke, no apparent effect). The second barrel functioned enough to fatally wound Abe at that range.
There have been some reports that Abe was hit with both barrels’ loads. No telling if it fired but not at significant velocity - home made/mixed powder tends to be inconsistent.