I go to the range to practice with two new revolvers; a 15-3 2", and a 586-8 4". I'am using WWB- 130 Gr. FMJ and Ultramax- 148 Gr. Hollowbase WC's in the .38 Spl. In the .357 Mag. I'am using; WWB- 110 Gr. JHP, UM- 148 Gr. WC's, L&B- 158 Gr. SP and finally Fed- 158 Gr. JSP. After about 45 minutes of firing I notice a serious smell of burning black powder. I can't locate the odor so I figure that it's just poor ventilation. After a nother 10 minutes of firing there is a full blown fire of black powder on the floor in the corner of the range where I'am shooting. Needless to say that the range is cleared and fire extinguished. By the time we're let back on the range everyone is furious with me and the owner tells them to knock it off and that this happens quite frequently. I've never seen such a thing happen before and hope someone could help explain what happened and what I may have done to cause such an incident. As always, still learning and Thank you in advance.