The second cannon seen is this little compilation is the 4 1/2 gun. damagellc.com/compilationvideo001.html We have some more video of that gun just have not loaded it to the site yet.
We managed to recover one projectile after it was fired at a refrigerator. It looked like you would expect the hollow point was ripped open with a few shards missing. We melted it down and cast another projectile.
In the US we can go as big as we want. But anything over .50 has to be registered with the government and has to go through a rigorous transfer process. It is a hasle but worth it in the long run.
Originally a large cordite grain was used. We opted to use modern 105 howitzer powder. It was a little slower burning than the original powder so we gave it a good black powder priming charge. We still get some unburnt powder out the barrel but I would rather have that than too fast of an ignition.
Some of the other rounds we have reloaded and fired: the 40mm bofors, 90mm US, 3 inch navy, 20 lahti, 25mm hotchkiss, US 57mm, 55boys, 20X81 German. Right now we are working on a 105 Krupp/rheinmetall.