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You can use a primer only and then press the case into wax to make a wax bullet. I'd be worried that a rubber bullet would bounce everywhere unless you had some kind of trap. Wax bullets have been used for some time. I think the problem with just a few grains of some kind of smokless powder is that you get too much "air space" and get more of a detonation rather than an ignition. Maybe if you use blackpowder in order to get more volume?
Primer only is likely to result in a stuck bullet. You'd be better off trying Trail Boss, Red dot, or something similar. Make sure that your bullet actually exits the barrel before you set off another round.