In this hypothetical, am I allowed to assume the power to amend the 2A or not?
If so, then amend the 2A to effectively end private gun ownership in the US.
If not, then create significant restrictions on the amount of guns any individual can own and the capabilities of the guns allowed on the market. No adult may own more than 5 total guns, no minor may own more than 2. All guns must be registered with the local government which rolls up to state and national registries. Criminal and civil liability for the use of guns in either crimes or unintentional harm to others is attached to the registered owner of the gun. All gun users must pass a significantly robust gun user licensing class before using guns and gun user license must be renewed every 5 years. Gun use without a valid user license is a felony with significant potential prison time. Magazine sizes are limited to 5 bullets and must include a mechanism that includes an outside tool (such as a screw or bolt) to attach the magazine to the weapon. All aftermarket products that increase the ability of the gun to be fired more easily or quickly are banned (with a small exception for ADA compliance that would be noted as part of the user license). There would be a transition period of a couple of years where gun owners could get licensed, sell or turn in excess weapons, and retrofit current weapons to the necessary restrictions. After that, violations of these laws would largely be felonies with significant jail time attached. Obviously, some details would have to be worked out and some adjustments made on pragmatic grounds as the changes are carried out, but that would be a great start.