I would use this in front of a dartboard - real steel-tip darts. Would a dart, dropped from, say 6' penetrate the rubber and make contact with my floor surface underneath?

You can use our 8mm Strong Rubber Tiles for this application. The dart would penetrate the tile, but would not penetrate all the way through to the floor underneath. The rubber tiles would act like the typical cork boards that are used around dart boards, but it would be better since it is more durable and would last longer.