If only I had twelve grand to spare

Then I could get a 20 mm rifle from Anzio.

http://www.anzioironworks.com/MAG-FED-20MM-RIFLE.htm

From the photos, it looks like they do gorgeous work.  And that video with the silencer!  lol  I'll admit that putting a silencer on a 20 mm cannon is not something I would have thought practical.  But American ingenuity always finds new ways to surprise me, it seems.  Major props to Anzio.

The downside to a 20 mm rifle is the paperwork involved.  If I recall correctly, these things are classified as "Destructive Devices," which means extra bureaucracy involved and long wait times.  But if you've got the money to spend on something like this, you can probably afford the patience, too.

Fun fact: Once upon a time, way back in the 1950s, Americans could order 20 mm Solothurns through the mail.  No forms to fill out, no background checks, no nothing.  Just simple mail order like any other product.  You could even pay C.O.D.

That all changed, of course, when the laws changed.  But there was a time when the only limit to your personal arsenal was your wallet.