Not a US citizen, and very much in favour of gun control, but I don’t think this argument holds much water.
Consider yourself a wannabe tyrannical dictator, but your population is heavily armed and might rebel against your takeover. How do you go about preparing your takeover? The answer: you slowly restrict gun usage. You make purchasing a gun more difficult, restrict the spaces you are permitted to carry a weapon, introduce buyback schemes to reduce the number of guns on the streets, etc., to ensure you have a monopoly on violence.
If you look at it from this standpoint, then the response is obvious; as a gun activist, you must vehemently oppose all restrictions on gun purchasing, because any restriction paves the way for yet more restrictions.