Agreed. Issue is more whether it would be an illegal taking under the Takings Clause. The government is not supposed to be denying you your legally-owned property without compensation.Guys, there is no requirement that newly outlawed items be grandfathered in, and a failure to do so does NOT violate the Ex Post Facto (XPF) clause of the US Constitution.
The XPF clause simply means that the G can't prosecute you for having OWNED (past tense) something that is now illegal, but was legal when you owned it. Similarly, they can't prosecute you for committing acts that were legal when you committed them.
If the G decided to ban all mags over 10 rounds and provided no grandfathering or buyback provision it would NOT violate the XPF clause.