Post by fightdesigner
I've done that with a modelgun uzi I used on a film set once- some cartridges have space for an extra cap, some don't. Half the time only one of the caps went off anyway, but occasionally both would. I was just doing it in the hopes it'd decrease the incidence of jams or misfires, since there'd be a 'backup' cap if the first one didn't go off, or an extra nudge if both did. Not sure if it helped in that way or not.
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Freelance hack... and slash, and gun.