Hi soeren,
I did some experiments to determine reciprocity failure effects and not only looked at long exposures, but also very short ones. Down to 1/250 sec of illumination, the negative responded at my rated exposure (EI 6 through a yellow filter for Ilford MGIV paper). with shorter illumination times, a 'reciprocity' like effect seemed to take place and therefore with a flash gun which has a very brief light, you may either need multiple flashes, or a much brighter flash than anticipated. I did manage to get successful images using flash illumination, but with my flash heads, needed about an extra 3 stops of exposure (8 flashes, each at the metered level). It was difficult to measure the exact flash duration unfortunately in order to generate a definitive behaviour curve for the paper.
Best regards,
Evan