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If a state like California (which has no RKBA in their state constitution) bans all guns, the people of that state may appeal to the USSC for protection under the second amendment. I doubt they would be successful because why? That's right. Because the second amendment only applies to the federal government, not the states.
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If a state like say, Montana, (which has a RKBA in their state constitution) bans all guns, the people of that state can appeal to their own state Supreme Court for protection under the Montana state constitution which states in Sec. 12:
"The right of any person to keep or bear arms in defense of his own home, person, and property, or in aid of the civil power when thereto legally summoned, shall not be called in question, but nothing herein contained shall be held to permit the carrying of concealed weapons."
Comprende?