There's already precedent for confiscation at an arbitrary amount specified by the government. Too low could/would trigger a 5th Amendment suit, of course...
"No free man shall ever be debarred the use of arms. The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government." -- Thomas Jefferson
9mm + 5.56mm =
.45ACP + 7.62 NATO =
10mm + 6.8 SPC =
Et Verbum caro factum est et habitavit in nobis; Jn 1:14