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Unfortunately, you have no case. Presently the law in Colorado requires that medical marijuana users are unable to have a concealed weapons permit. The justification behind that is that marijuana is illegal under Federal law and regular use of illicit drugs will cost a person his Federal firearms rights.
There's a movement to try to change Colorado's law to remedy this, and the possibility of such an amendment is up for a vote on the 2016 ballot. You can read more about this here.