How do you own C4 legally?

Answer 1

The likelihood of you being able to own C4 for non-business purposes is slim to none.

First, you must obtain a Destructive Device permit (similar to the process of obtaining an NFA regulated firearm).

The application must be approved by the BATFE, your state's law enforcement, and your County Sheriff's Office.

Most states - including those which regularly approve NFA permits - will not permit anything classed as a destructive device (firearms over .50 cal, explosives, etc.) - or else will only allow large bore rifles, and the only exceptions for explosives are typically restricted to businesses which require their use for the purposes of that business.

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Agree with above. In the US, you must comply with the provisions of the Safe Explosives Act to possess explosive materials. This requires licensure with the US BATFE as a "Responsible Person" and as an "Employee Possessor" to purchase or possess explosives. You are then required to properly store the materials and to account for their use. Your state will also require you to be licensed by state authorities to use explosives. This is one of the most heavily regulated products in the US. Writer has been a licensed explosives engineer since 1974.