In WW Greener's book, he talks about 4-Bore double ball guns. Secifically he talks about a 4-Bore double with 20 inch barrels, a charge of 12-14 drams and a 1510 grain spherical bullet. I was wondering if any of these Greener 4-Bore ball guns have survived today? I have seen a single 4-Bore ball gun before (not a Greener), but have never seen a double. I'm sure they are out there. Any photos or information would be great. Only looking for photos of double ball guns, not double rifles.