I know someone who was very concerned about this so in the filling cap of his cylinder flask he drilled some farly large holes in it then covered the holes with gaffer tape, the theory is that if a spark somehow got into the flask the resulting gas expansion will exit from the weakest point, that been the gaffer tape in the bottom of the flask which is now upside down and at about 45 deg to your body.
I now make paper cases with 22 grains of black and a filler (fine semolina), this makes loading much faster and cleaner as you make them up at home.
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