David,
These German barrels were very likely bored tighter than the 0.729" bore dimension normally associated with a 12 gauge. This is typcical of many continental guns. If you look on the barrel flats you may see markings indicating its tighter original contriction - e.g., "13/1" in a circle. So your choke contriction calculations are very likely off if you used 0.729" as part of the equation. There for your chokes may be more open than ou think.
I regards to proper ammunition, you didn't mention the gun's chamber lengths or how the gun is marked in reference to proofing. As log as you stay within the bounds of what the gun was proofed for you'll be fine.
Good luck,
Ken
